The ones used in your current project or all installed ones?
a) For the ones in your project, you probably would need to read them track by track(TrackFX) and item by item(ItemFX) using the accompanying API-functions (they probably have an FX in their name).
b) The ones installed can be accessed in the following config-files, I documented:
VST-Plugins:
http://mespotine.de/Ultraschall/File...tplugins64.ini
Reaper-vstplugins.ini is for 32-bit-VST-plugins, but should work the same way.
DX-Plugins(Windows):
http://mespotine.de/Ultraschall/File...xplugins64.ini
Reaper-dxplugins64.ini is for 32-bit-DX-plugins, but should work the same way.
AU-Plugins(MAC) are registered in ultraschall/ReaperConfigZip/reaper-auplugins64.ini, though I haven't documented them yet.
Maybe a reaper-auplugins.ini has the 32-bit-versions, but I don't know, as I have no Mac to check...
You can read them and parse all names out of them, using pattern-matching.