This is a beta only so I'm going to skip straight to the goods, Treo Keyboard should be supported, it works on the 700p and is enabled by default if the device shows up as a Treo. I might have to add the Centro I am not sure it is in the list.
I wouldn't recommend this to new users, check out PalmMAME 021 first.
Now How do you use the keyboard? Well once you launch a game then you open the menu and start assigning whatever you want to the keyboard. The easiest way is with the StatBar. Use the keyboard just like the PC keyboard in MAME. This doesn't affect your keyboard setup in the launcher, you still need that at least in a minimal form, and it is the way you need to access the non letter keys anyway.
So try it out, if it works I'll start putting together the whole package properly.