Depends on how many mercs you have handy, but a route to quick cash if you can see a way is generally to throw a BIG army at an enemy king. If he can't win he'll abandon the province rather than hole up. Raze the province for cash, then retreat with the winnings. If you can't afford to train new troops, now might be a good time to "rationalise" your training provinces. Keep the really good teched up ones and your valour bonus provinces should be stripped down to produce ONLY the "bonus" troop type. How's your navy? If you have control of the seas deploy the ships for max trade benefit and "loyalty links". If you don't have full control, hit enemy navy ASAP so you do have I agree the time is probably ripe for you to help yourself to some of the Pope's baubles. If you're excommed already, what do you have to lose? Of course, assasinating the Pope to get re-commed would help your loyalty levels and free up forces to use elsewhere.

Whatever you decide, you'll be able to look back and say "that was an INTERESTING period in the Empire's history"

Also as highlanddave says, it's probably a bad time to send the king into battle. You can't afford any more loyalty knocks right now. Any besieged allies you can rescue - that's always good for an easy influence boost.