diode selection for large current reverse bias, small current forward bias
What am I looking for on the diode spec sheet to be sensitive enough
  to allow the low current forward bias and strong enough to block the
  large current reverse bias. I know that is what diodes do, but how to
  select the right one escapes me.


Just use a high speed diode like a 1N4148, BAS16 or 1N914. It will certainly block 12 volts and won't conduct any appreciable reverse current when the heavy duty 12 volt supply is activated. I said use a fast diode because reverse recovery time for those stated above and in the single figure nano second range.