How to hammer in tight place
No, at least not that I've come across. Sure, you can find bars shaped like that, or make your own, but they're not readily available in tool stores. 

Your best bet is probably to try and pry the panel down rather than hitting it with a mallet. Use a flat bar or large screwdriver or other tool along with a block of wood to protect the edge of the panel. Exactly how you do it will depend on the particulars of your situation. 

Update: Mazura reminded me about a flooring installer's best friend, the Z-bar. Might do the trick, but at a cost for one little task.