This strategy has developed into a proprietary, but probably more elegant approach, to avoid the overengineering of Base85.
Essentially, I've written a routine which pushes the extra bit per byte into an overflow byte which is then written (at most) every 8th byte. This is immediately more efficient and comprehensible than Base85.
You can see some more discussion at the relevant issue on PyFirmata github, with some example (untested) code...