Novel use for Marlin + RAMPS 1.3, help needed

I recently bought an '80s 'vintage' robot arm, that was designed for educational use. It's a 5-axis (though 6 motor) Armdroid, and came complete with the original electronics. While I plan to resurrect the original electronics for historic/sentimental reasons, they are limited to only moving one axis at a time, which just isn't cool. So, with a bit of hacking I hooked up the 6-wire 7.5 degree NEMA23 size steppers to a RAMPS 1.3 board, with them connected in bipolar mode. I found some old datasheets for the motors, worked out a rough stepping rate that worked etc, and have reasonable motion out of them. It's controlled by Pronterface at the moment, and all I really want to do with it is use it as a demonstrator, so can hand-craft gcode to get it to move things around. I have control of the all of the axes - two motors are tied together in parallel to control the 'wrist' movement, which is why a 5 axis board is enough to control it. The wrist movement is the 4th (E0) axis, and the gripper is the 5th axis (E1)
I'm using Marlin v1 firmware, which gives me the control of all 5 steppers. However, as it expects the 4th and 5th axes to be extruders, and to swap between them and use them independently, you have to send a tool change command (T0 or T1) to switch between the two. Then there is only one count for extruder position, so if I close the gripper and switch back to the wrist axis, the wrist axis will have lost it's position.
Question: is there an easy way in Marlin to support the 4th and 5th axis independently? Obviously this requires a little firmware hacking, but as there don't seem to be any other firmwares that support 4 or 5 axes, it seems the easiest to use. I think grbl only supports 3 axes, otherwise it would be the ideal choice. I'd quite like to continue to use Marlin, as I may eventually replace the gripper with a paste extruder, so keeping the extruder control makes sense.
Any help/advice greatly appreciated.

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