Thanks for sharing your insight!
I now understand why I sometimes see one instance of regsvr32.exe in Task Manager / Processes, and sometimes see two. The default Image Name window happened to be just narrow enough that I didn't see the *32 at the end of one of the names until I widened the window.
I still don't know why one of my dll's is causing regsvr32 to hang, but one mystery is solved.
Thanks for sharing your