thanks, but its not a matter of consideration. The enclosures are drilled, painted and printed, the hardware is set, PCBs done, components soldered. It's all done and dusted except for the final assembly and, except for this significant add-on feature, if it can be made to work.
The software of course can still be changed, and some minor hardware can be added to the supplying enclosure, provided it doesn't take up too much room and it is physically stable enough when soldered and hanging of a pin on the TRS connector. In that scenario, a small current limiting resistor is OK.
It doesn't sound like there is much else that can be done within the supplying enclosure.
There is plenty of scope for hardware changes in the receiving enclosure.
I know this might not be the usual way, but what if I sized the limiting resistor such that half the voltage was dropped across it and used a 40mA charge pump in the receiving device. I don't think the current for the uC and opamp circuit varies much, so things should remain stable. If the uC and opamp circuit is drawing 40mA, then can I assume the input resistance is 125 ohms?
Do charge pumps only double the voltage or are there fractional options.
If so, then I size the limiting resistor at 125 ohms/ 250mW and use a 40mA charge pump in the receiving device. Unorthodox, but would it work.