Doesn't help monkey boy with the dislocated shoulder, but i'm finding leg-raises pretty good for the old abs.
As per philos comment - hang bar at 90deg lock, raise legs keeping them straight and keeping the lock and back in good form, lower and repeat.
When that gets easy hang weight from your ankles and continue.
When that gets easy start aiming for the front lever.
When that gets easy, congratulations - you are now nails.
(i am still at stage one of this programme
)
Also, similar exercise on bar, but one leg at a time, cross right leg as far as poss out left, curling left leg under the body - sort of a fly-kick motion (but real slow) then repeat with left.
Key to both these is doing it slow controlled and concentrate on keeping good form throughout.