Tempo and scoring to video

I'm using Sonar Producer X3 to write a score for a short film. I have the video imported and have already written a few parts at various times. Now, I'd like to be able to use the piano roll grid and metronome as I write new parts. Unfortunately, if I insert a tempo change in the middle of the project, anything after that point is thrown way off sync with the point in the video it's supposed to occur. I'd like to be able to lock parts I've already written to a certain tempo and start time. Can anyone think of a nondestructive workaround for this?

You can lock the start time of a clip by switching to H:M:S:F instead of MBT in the clip inspector and setting an absolute start time, but that may defeat the purpose of writing a score. After you do the score, the musicians who are interpreting it are unlikely to produce a frame for frame match anyway. Do you plan to do some time stretching after you have the score recorded?

Scratch that, I think I figured it out:
Put the cursor at the beginning of the next chunk of music you don't want altered. Make a note of the tempo at that point
Ctrl-A, then Edit | Select | From = Now
Edit | Cut Special, and select Events, Automation, Tempo changes, Meter changes and Markers
Create a marker at that position and lock it to SMPTE
Now you can backtrack and make tempo changes.
Put the cursor at the new marker
Insert tempo of whatever it was before
Paste.
 
Everything SHOULD retain its exact timing with this method. It won't be in the same position relative to the grid/meter, but that suits my process fine!