Is there any way to get the Sonar arpeggiator to have fine, non-fractional control over an arpeggio rate? With automation? I'm trying to make a gradually increasing arpeggio effect, but Sonar's arp and every arp plugin I try seems to be locked in to the project tempo.

fine, non-fractional control over an arpeggio rate?

 
It's possible to 'embed' that within a pattern file (*ptn), then use that as a basis for the arpeggiator preset.  You can check in Sonar's included PTN library [... \Cakewalk Content\SONAR (version)\PTN MIDI Patterns], but that may be an exercise in futility.  I used to create pattern files for .arps in Project5, and I believe that I did a few "downshifting" presets along those lines.
 

With automation?

 
It will sweep through the available beat divisions.  That may be enough with the dotted and triplet variations, but timing the transitions between divisions will be critical.  It's not going to be as smooth as a sweep in frequency would be.

Hmm. Anyone know a VST that can do this? Closest I could find was Arpimedes, which is non-sync but goes mute while the rate's being automated.

Maybe this one ?.
 
http://hgsounds.com/product/arp-vsti/
 
Has a "Manual Tempo" mode.
 
Haven't tried it myself - let us know if you do :-)
 
Cheers - Cliff

I use Sugar Bytes Thesys less than I should.  Full featured arpeggiator, and regularly discounted to $29 US at Musician's Friend.  But I believe it will only slow down (handbrake), and not gradually increase in rate.  The other suggestions below aren't standalone arpeggiators, and may not apply if you're "married" to a particular synth or sound driving the accelerating arp.
 
You might consider the original z3ta+ 1.5 included with Sonar, or z3ta+2, if you have it.  I know that you can disengage tempo sync, and put it in Free mode.  I've used MIDI Learn over Speed in the original, and I'm certain that the Arp Speed parameter is exposed to automation in z3ta+2.  Both have Arp Speed as a destination in the MIDI Matrix.
 
Not quite the same in Rapture (sequencers vs. arpeggiators), but it might work for you - depending on the phrasing.  Rapture's sequencers can cycle to frequency, or to tempo sync division.  Again, exposed to automation or MIDI Learn, and controllable as a destination in the Modulation Matrix.
 
Rapture can also be set to an Internal Tempo (in Options), which can be 'tapped'.  It will ignore system sync, but no automated, external control over rate for this kind of tactic.