Mark, I've taken (literally) thousands of photos of
foxes with flash. In my experience if they tolerate your presence, then the flash isn't a problem.
The main problem is that from a distance you'll get 'headlights' rather than eyes. So it's good to be in quite close so your idea of using slave flashes sounds good (I've not tried it, but the principle is sound). If the angle of the flash is right and the flash is with 10 feet or so there's a decent chance of no eyeshine (upward light is more likely to cause eyeshine, so angling down my be better. But experiment if you have the chance). Otherwise it varies from near-white to a lovely deep turquoise.
If the eyeshine is 'clean' (i.e. just the pupil area) it's easy to fix in photoshop. The technique I use is to mask the eye and then use the Enhance Color>replace color options. I use the eydropper to select the dominant colour and then slide the lightness control to the left. You can play around with the fuzziness settings (I tend to run between 130-180). Sometimes a couple of goes is needed if you get a multicoloured eye. Here's a recent example.
Before:
After:
