Ben Stragnell wrote:

> It would appear that I need to be able to place the SX's OSC1 line at
> 0V, 5V and 12.5V, all under control of the PIC - and since I'm a
> programmer by trade, not an engineer, I have absolutely no idea how to
> proceed!

  Well, for hints, I'd look at this circuit for starters:
http://www.dontronics.com/graphics/dt001cct.gif

  It contains the driver to pull the voltage up to 12.5V or let it drop.
You could pull it up to 5V with a resistor and diode (or perhaps, just
with the resistor) and pull it to zero with a gate of the 7407.

  Incidentally, if you prefer a 74LS07, make sure it's a T-I SN74LS07.
Other brands are not rated for this job.
--
  Cheers,
        Paul B.