It has occured to me that there is a stealable source of -10V on the pins of my MAX202 ECPE chips. Are there any pitfalls, apart from the obvious don't try and run more than a couple of mA from it?

Hallo Steve,

there's no problem to do this! If you have a look at datasheet of MAX220 you will even find the following lines:

A small amount of power may be drawn from the +10V and -10V outputs to power external circuitry, ...

Filtering is essential of course. I would use a RC low pass filter of let's say 100Ohm + 10µF Tantal. If you omit the resistor, your 10µF Tantal will be involved with charge pump process of MAX202E. That would cause spiky currents to flow between the ground point of Tantal and ground point of MAX202E. But if you use this filtering resistor, you can drastrically decrease ground noise between the two ground points. This is essential, if your additional circuitry which uses this -10V is a bit separated fom MAX202E and needs to be decoupled by this 10µF Tantal.

Good luck,
Kai