Especially in the context of media players with plugin support, it may be desirable to explicitely prioritize one plugin over another for specific formats, e.g. because libopenmpt and another plugin handle formats X and Y, but while libopenmpt handles format X better, the other plugin handles format Y better. XMPlay solves this problem with a list of priority file extensions, but it would be nice to solve this problem explicitely on the format level, and not the file extension level (as a file extension can be bogus).
There are also subsets of formats that we may want to block, e.g. S3Ms with AdLib instruments, while still allowing to load purely sample-based S3Ms (though this is no longer as relevant as it used to be, as we have built-in AdLib support now).

This could be solved through a ctl, or by using the probing system. For example, a format ID could be returned.