Anyone have a source for a schematic for a 12v around 20 to 30 amp
power supply that makes use of a full wave rectifier? I've found quite
a few for a bridge rectifier but I'm not really sure how to mod them
to use a full wave rectifier.

Hi, Bridge rectifiers are full wave. Sounds like you want a full wave center
tap arrangment. These are very common, surprised you could not find a
schematic.
With a 32 volt center tapped xfmr, connect the center tap to common and
attach 2 rather large diodes to the other two transformer leads, and common
their anodes. You now have an unfiltered 22.6 volt full wave DC power supply.
Xfmr CT is negative, and the diode anodes are positive.
To get to a 12 VDC regulated supply add sufficient filter capacitors, and
regulator devices. A 12 volt 20 amp linear power supply requires heat sinks on
the solid state devices. I use about 50,000mfd filtering on mine, and a 200
volt 50 amp darlington transistor, as the series pass transistor regulator. I
used them because that is what I had on hand. The final design of your
regulated supply will depend on a number of factors.
73 Gary N4AST