Simple two wire connection to a programmable Honeywell RTH6360 themostat. Have a 'C' wire in the 18/5. Do I still need batteries?
The manual says "C does not power the thermostat display or operations; batteries are always required", so there's no reason to doubt that.

Putting 3V on a pair of wires and running them to the battery terminals would probably work fine, but obviously it isn't intended and I have no idea if it's to code.

Less complicated options would be to just put batteries in there (they should only need replacing every couple years) or buying another thermostat that is intended to be powered by 24VAC from the R/C wires.