Wifi Thermostat to Reznor Garage Heater. No "C" wire connection on terminal strip
After more carefully reading your question:

Neither G nor W1 are suitable to use as a common (C).  The C connection is there in the unit as you said, but not present on the rear terminal strip.

Based on the manufacturer schematic, it does appear that "W2" on the rear terminal strip of the single stage unit is truly not connected to anything, so you should be able to extend your "C" terminal from the interior circuit board to W2 on the rear terminal strip without any adverse consequences as you mention.   It would probably be a good idea to relabel "W2" on the rear to "C" as well.