I'll admit I haven't read through every post here, but based on the last one..
You could always (using on error resume next) set the .installed property to false of the addin, then if you wanted you could even remove it from the list via the registry
hkcu\software\microsoft\office\x.x\excel\add-in manager
At least in my version (9.0 / 2000) they are listed there. It looks like there are some user-specific ones too though, not quite as easy. I believe the ones there are the non-installed ones, and the installed are listed under software\microsoft\office\x.x\excel\options with a name of OPEN to OPENn (ie OPEN1, OPEN2). I also believe these get updated when excel opens/closes, so you might have to look under \options

Though I suppose these are just rambling thoughts, take it at face value