edit - Jane beat me too it.
These are awesome and cheap! They just slip onto the door frame. Not permanent. No tools required to put it up, either.
http://www.getirongym.com/Default.asp?bhcp=1
They even sell them at Target, here. $29.
Pullovers aren't really a good substitution for chins/pulldowns. It's mostly a pec exercise, but you get some back in there.
http://www.exrx.net/WeightExercises/...BPullover.html
I would try to get the chinning bar, but until then do some extra db rows with a different set/rep/grip scheme.