Here's something to try... www.un4seen.com/stuff/libbass.a
It adds support for passing the name of an add-on to BASS_PluginLoad. For example, like this...
BASS_PluginLoad("BASSFLAC", 0); // plug in BASSFLAC
BASS_PluginLoad("BASS_APE", 0); // plug in BASS_APE
There may be a problem with loading plugins in this way though, because a linker generally won't import a library if it doesn't appear to be used. For example, when the "BASSFLACplugin" symbol is used, the linker knows that the BASSFLAC library (libbassflac.a) is needed because that's where the "BASSFLACplugin" symbol is located, but it doesn't know the BASSFLAC library is needed if a "BASSFLAC" string is just passed to BASS_PluginLoad. Hopefully the MonoTouch linker won't do that. If it isn't working (BASS_PluginLoad fails), you could try also adding a BASSFLAC function call to your app (eg. call BASS_FLAC_StreamCreateFile with an empty filename) to force MonoTouch to import the BASSFLAC library.