Adam,
Yup, github pull requests are ideal. If you're super allergic to git for some reason, I suppose you could email us patches at info@leaflabs.com, but that's a lot less convenient.
Not sure what you mean by "improve this"; I'm extremely impressed with the convenience github has provided thus far.
UweBonnes,
A _lot_ has happened since 0.0.9. All of our development work is currently taking place against the "refactor" branch in the libmaple github repository. Patches to the low-level pure C code will almost certainly not apply. We're grateful for your patches, but would encourage you to work against the current development code if you're going to be contributing patches for the time being.
The refactor branch has a preliminary design document here:
http://static.leaflabs.com/mbolivar/libmaple_design/
In particular, the hard-coded clk_speed (along with the rest of usart.h and usart.c) have been largely rewritten to be more general. I have yet to push those changes (they're in my local clone), but am hoping to get them in by the end of the week.