Potseal, you are under a misconception.
What limits voltage drop issues to branch conductors alone? Circuit or panel - you need to allow for voltage drop. Only a higher voltage will lessen your voltage drop.
Set the 'percentage rule' aside for the moment. What's your final voltage when the heater is in use? If your incoming voltage is on the high side (127V is not all that rare), then you could very easily have a very acceptable 118V at the heater.
Whether you should set a panel is another question. I'm biased in favor, for two reasons.
First, I like having an OCPD, GFCI, and disconnecting means close to the point of use.
Second, you almost always have a later desire to add a light, a service receptacle, whatever in the same area. Might as well pull a 4th wire and have a second circuit.
Run big pipe - pipe is cheap - and you can pull bigger wire later if you need it.
Finally ...don't have 480? Make it. Just use two transformers: one 'up' and one 'down.'