I think I may have answered my own question.
The power supply in the chassis appears to be a Servo Dynamics 4017pm "DynaPower".
If my unit matches the manual like I think it does, it accepts power input between 45 to 120 volts AC single phase and supplies up to 6 servo amps with power. DC voltage is 1.4 times the AC input voltage.
It looks like from the wiring in my mill that 110v was still being supplied to the servo chassis for some reason, so I'll look into why that was. Apparently this power supply gives the drives all they need from the AC input voltage alone.
I'm thinking I'll re-wire the transformer to supply power to this chassis, and use a 220v feed to power the PC "brain" of my mill, with 24v for some other functions like contactors. The only thing remaining three phase will be the spindle motor, which I'd like to put on a VFD.
Any further info on this power setup or confirmation of my assumptions above is welcome.