It occurs to me that if I pull the output of the transformer using half the secondary, it increases the ratio to 15:1.  This would allow the 1.5A input to result in 22.5A output.  This would limit the voltage to be 0-7.5 VAC RMS, though the higher current would only ever be needed with thicker gauge wire, so lower voltage required.  

I could add a switch to control which transformer output to use as one option.  I'd need a high current switch for that, and to increase the bridge and relay current ratings.  And if measuring the current, the existing shunt would be dumping 5W of heat at max current, which is it's max rating.  I suppose I could bond it to the chassis as a bit of a heat sink. (That configuration also means the meter would be maxed at 19.99A). I don't have a 20ADC breaker, so it would rely on the input breaker; I'd need to switch out the 10A one.  Or maybe get rid of the output breaker as redundant.

Alternately, I could add a separate parallel rectifier and separate outputs, though it would then bypass the metering.  Might be easier to switch it's inputs though.

After some thought, I'm going to implement this.  I have a bunch of 35A bridge rectifiers, and a 25A switch and relay.  When using steel or other lower resistance wires which I may want to do at some point will require lower voltage and higher current, so this will give extra flexibility.

For switching modes, the switch will be move the output side common between the bridge negative and the transformer center tap.  Doing this will reduce waste heat in the bridge, and more evenly load the transformer output windings.

Switching this way changes the rectifier circuit between these two configurations:

Full wave center tapped rectifier:

Full wave bridge rectifier

For the center tapped version, I can use the same bridge rectifier. Only 2 of it's diodes will be in use which will reduce it's power dissipation; good given the higher current in that mode.  The bridge is rated 35A, however the heat becomes a limiting factor as I will only have the steel case as a heat sink, and no airflow.

I'll remove the output breaker as redundant as the weak link in the chain really is the variac at the input, with the output side having margin.