I have attached two perl scripts that I wrote to make instantaneous transitions to/from MathAction and Axiom input files. I attached the example file quatsandbox which was copy/pasted from your sandbox and used in the demo executed below:
~/axiom/perl$ ./mathaction2input quatsandbox quat.input ~/axiom/perl$ ./input2mathaction quat.input quat.math ~/axiom/perl$ diff quatsandbox quat.math ~/axiom/perl$
As usual, you may have the edit the first line of the scripts to point to your system's perl binary. Or you can instead invoke with:
perl mathaction2input quatsandbox quat.input
These are relatively simple and do just what is needed for the translation. I admit that with another hour or two of "optimization" I could make them have half as many lines of code and half as readable, in the classic perl style, but I don't see the point.
Please feel free to attach to the MathAction site where appropriate and otherwise distribute.