I've now put the .dll back in their original folder, but Sonar still crashes almost immediately on startup.

Bill, in principle, I like your solution way better (I don't want to have to run SONAR as administrator in general, though I believe there may be a few other plug-ins that have issues if you don't, too -- maybe SampleTank or something from memory?).  However, when I try that on my system under X2 Producer, I can't get Pentagon to make a sound (it loads fine).  Even more significantly, once it is loaded, other softsynths also don't make a sound.  In fact, it seems to kill the audio engine entirely -- even playing an audio loop didn't work, with the transport's basically not wanting to start.  Thus it seems like something with Pentagon I running that way is causing a permanent state of dropout (it actually does light the dropout light in X2 Producer, but not in X1 Producer Expanded).
 
Rick

Hmm.  You mentioned that you are scanning your DXi folder.  Try removing that.

I can open and run Sonar as long as I don't open the project that has the Pentagon synth inserted. That project crashes immediately.

Open the project in safe mode by using SHIFT.  Remove Pentagon and save the project.  Re-open normally and try re-adding it.

OK, this is too weird. I dragged the entire Shared DXi folder to my desktop, then started the project which has the Pentagon insert, and was able to access the project. I then deleted the synth, saved the project, and exited. Then I put the DXi folder back where it originally was, and re-started the project. Pentagon appeared in the list of browser synths (not in the browser VSTs, as it was previously), so I crossed my fingers and dragged it to an audio track. It's a bit early to say for sure, but it appears to be working as it should. Go figure.