Thanks for the request.
We are working on an ARM version of TinyG that will support external axes (and other function) via the Kinen pluggable interface:
https://github.com/Kinen/Kinen
One possibility if you need something sooner is to use the Arduino Due port of TinyG under the G2 github repository. Bear in mind that this is really a development project and experimental at this point. It works pretty well, but not all features are implemented or tested. The Due version can be found here:
https://github.com/synthetos/g2