The Fit to Time... option in the Process menu should give the OP the desired results.
 
1.  Select a MIDI clip.
2.  Set the Time Format in Clip Properties to H:M:S:F.
3.  Set the Time Base to Absolute.
4.  Determine the current End time of the Clip by adding the Start value to the Length value.  NOTE:  Verify the Timecode Format field on your system in Preference > Project > Clock, since it affects how we add the Frame fields.
5.  Change the tempo.  At this point the clip should still have the same Start value as before the tempo change.
6.  Run Process > Fit to Time...
     a.  Click the Format button in the dialog box until the time format is H:M:S:F
     b.  Set the New Thru: field to match the End time calculated in step 4.
     c.  Click the Event Times radio button.
     d.  Click OK
 
The MIDI clip and all notes should still be at the same absolute SMPTE time as at the start of the procedure.
 
Hope this helps,
-- Ron

Yes. Fit to time is likely the better answer. I knew there had to be an easier way. I just couldn't think of it.
Doh!!!

Base 57
Yes. Fit to time is likely the better answer. I knew there had to be an easier way. I just couldn't think of it.
Doh!!!

Fit to Time... wasn't the first thing that came to my mind either, since I have never used it on a project. 
 
It might help us if the OP explained the purpose of having the MIDI at a fixed time.  I was trying to imagine why I would want MIDI notes locked to a specific SMPTE time and could only come up with triggering something such as sound FX at a specific time in a video or stage lighting control.
 
-- Ron

Obviously, Fit to Time was the first thing I thought. But it does not work either. Your procedure is valid if the original clip does not contain tempo changes inside (only one metronome value from start to end). But if there are tempo changes in the clip, this process erases them (in my clips tempo map there are hundreds of tempo fluctuations and I want to eliminate the changes in the tempo map without eliminating the actual timing of the clip). Simply, the lock command (absolute time) should lock the notes in their current absolute (H:M:S:F) position (and then if you change tempo map, their timing does not change, only the notation in bars change). According to the online help this is possible and simple, but unfortunately it does not work. Or maybe I do not understand the logic of the program ...?

Sorry if it's a little abstruse. I try to explain my problem.

1) I get midi clips recorded metronome, very rigid, like printed music
2) I must 'humanize' the clips, inserting many tempo changes (rallentando...accelerando...). In this way, each note moves to new, different SMPTE positions (H:M:S:F). 
3) Finally, I have to eliminate the changes made in the tempo map, but I have to keep the new 'humanized' timing
4) To do this, I used to lock the absolute positions of the notes after editing the tempos and then erase all the changes in the tempo map. Thus, there is still only one metronome value at the clip's beginning, but all the job of 'humanization' is saved. Every Midi event remains locked in the new absolute position in which my tempo editing has shifted it. Musical time (bars, rhythmic notation) is changed, but this is not a problem.
 
That's just what I can not do with Sonar ...

Thank you for the explanation.
 
It appears that in your process, that the MIDI clip would have to be set to ignore tempo changes.  I am not aware of any process or setting in SONAR that allows that.  Maybe someone else does.
 
Sorry I couldn't help,
-- Ron

Theoretically Sonar does it (lock the absolute time of midi events, as you can see in the manual), but in practice it does not work.
Thank you very much.

Actually the quoted material from the manual is fatally ambiguous. What it says is that the position of the clip should lock to absolute time. Since a clip clearly has a start point, a duration and (determined by the duration and start) an end point, one would assume that the position is most likely the start point. That does not specifically state that the duration of the clip will not change if the tempo changes. So the only thing that you can count on as far as the manual is concerned is that the start point (presumably in HMSF from the project beginning) will not change with tempo changes. It would certainly make logical sense that a locked clip will not change in any way until the lock is removed, but that question is not addressed in the quote, and there is no specific language that would indicate that whatever behavior is to be expected will differ in any way between an audio and a MIDI clip. 

The manual seems quite clear:
 
"If a clip’s position is locked, and you change tempo, what happens to the clip’s position depends on what option the Clip Properties Time Base field is set to: Musical (M:B:T), or Absolute (SMPTE). If the clip is set to the Musical time base, the clip’s M:B:T position stays constant, and its Absolute position shifts. If the clip is set to the Absolute time base, its Absolute position does not move, but its M:B:T position shifts."
 
This seems the typical operation to lock Midi events at their SMPTE. But actually it does not work, notes are not locked. No hope, I fear...

I agree that what is stated in the manual can be confusing. At first, I also interpreted it the way you have. But we were wrong. It says "what happens to the clip’s position depends on what option the Clip Properties Time Base field is set to".  By position they mean the start of the clip. A MIDI clip will always follow the tempo map. I checked X3, X2, X1 and 8.5 and those versions behave the same way as Platinum does today.
 
However, the method I described in posts 18 and 20 works. Step 4 is a little time consuming but you either want to get the job done or you don't. 
 
A 5th step to complete the work is after applying the new tempo re-select all of the locked clips, cntrl-click unlock and then bounce all of the separated clips back into one clip.
 
If you have access to another program that does this more easily then you should use it.
 
Best of luck to you