What relay does a C wire toggle? From what I've read its only purpose is to provide a continuous stream of power. There's nothing in the spec that forbids using the R and C wires for power, in fact that is their stated purpose.

Now, these bypass kits or similar, are a little hacky. But you're talking about the C wire itself being so, which just isn't the case. IF you have one available that is exactly what it is design to do.

But, yes, I'd prefer USB at the thermostat.