@Chris
Hmm, the more I think about this the better it sounds.
AnnouncePlayer would be expanded to also return an array of formats (probably limited to 4 characters, like IFF chunk names). The TuneNet website would act as the format registrar, but most formats will be the common shortened names (S3M, XM, MP3 etc). This information could be shown in the plug-in about boxes which appear when you double-click plug-in names on the list.
TuneNet would collate this information as it loads the plug-ins, so it can reverse look-up plug-ins against a format (so XM would return TN_XM.tnplug, TN_XYZ.tnplug etc), and create a list of formats in prefs which has a cycle gadget next to each one where you can select your preferred plug-in (the default would be "first available" or perhaps "ask")
TestPlayer would be expanded so the plug-in also returns the format of the file (this has the added benefit of resolving my other request if TuneNet displays this as part of the format field in the file listview). TuneNet would then check this against the registered plug-ins list for that format, and attempt to use the user's preferred plug-in first.
If it has no joy with the new-style plug-ins it scans the old-style ones next as per usual.
Come on Bean, I've practically written it for you