The obvious solution to this would be to cycle randomly between fonts every few characters (or keep a running total of the font used for each particular symbol,

The obvious solution to this would be to cycle randomly between fonts every few characters (or keep a running total of the font used for each particular symbol, and ensure it stays below some threshold). This sounds like a useful defence in general against OCR re-use of particular things you might publish. I wonder if it could be done in a manner unobtrusive to the eye, but progressively more expensive to algorithms, either in terms of memory or time. This is really a neat idea you have.