The Mainline 6.3V rails are biased up from ground/earth by about 50V, so that could very easily damage your RP or if you had a USB cable plugged into the RP and you touched the metal shell on the other end, you'll get a zap.
There's also not a ton of extra current available in the heater circuit, so without knowing the current draw it's tough to say if the Mainline would be damaged.