Yeah, you can only override source action files. So when class files take over and do the work (and especially when they redirect, etc. afterwards) it makes things a little challenging.
Some class files have built in hook functionality (like publicSessions.php) but not all.
I think you'll have to modify this for now, but we will look into building in specific hook functionality for the posting library in the next update, as that's very useful functionality.
WildRAID reacted to bfarber in Hooks in the middle of posting process?
