I am trying to add some functionality to Pro Tools, but the only way I have figured to get a hook into the system is by making an audio plugin. Normally a VST allows you to add processing to some audio, but is there a way to hack it so that it actually saves the incoming audio externally instead of (or in addition to) adding processing?
Yes. The plugin is fed the audio in chunks and could save them to disk. Based on the routing in the DAW it sees only part of the total mix, unless it's a master plugin.