Thanks guys. I guess there are really two hurdles..
1) Being able to manipulate the list itself. Feedback I got from Facepunch is pointing me at the "downloadables" string table. Looking at the dump does show me the gma's I expect to see. Assuming I can edit this list on the fly (the stringtables module was mentioned, but that looks like its read only) then that's a potential solution to this.
2) I still may run into mounting issues on the client side. AddWorkshop() doesn't just instruct the client to download the addon, AFAICT. It also instructs the client to mount it. If I reduce my AddWorkshop() calls, those addons I don't call do not get mounted, even if the client already has them. Manipulating the downloadables stringtable may very well create the same issue.
In theory I suppose its possible that I could call AddWorkshop() with all my addons at server startup, and then trim down the list with stringtable manipulation **without** breaking the mounting functionality for the client. But this assumes that the mounting list ultimately gets separated from the downloadables list, and that's a big... big... BIG assumption. Not something I can test until I understand if actually changing the downloadables stringtable is even a possibility. Is there something other than "gm_stringtables" (
https://github.com/gmodcoders/gm_stringtable) that might be considered the "stringtables module"? I haven't discovered anything else similar, and certainly nothing that seems to allow write access to the stringtable.
Thanks again.