The RDTC board has the 3.3v regulator on it to convert the servo +5VDC to 3.3v for the TC amp. There is also a jumper that allows you to bypass the 3.3v regulator and run the TC amp at +5VDC, that is where you will get a slight offset. Theoretically you could use one of the extra wires in the CAT5 cable to send 3.3VDC from the HM board to the RDTC board, then jumper the 3.3v to the solder pad to bypass the regulator, which is I think the answer to your specific question. You are right, you will lose a food probe in doing that. I have been running a RDTC board with the regulator for many months now and it works great, so I don't see a reason to dedicate a wire to 3.3v.
On another note, a benefit that you wouldnt expect and most people would never run up against... I leave my RD out on my grill and powered by my HM 24/7 to give everything the torture test. During development of the RD3 I had the control box so it could only go on one way, and on my grill the CAT5 jack pointed up with that configuration.... As a result rain would pool into the CAT5 jack and shorted out my RDTC board a couple times. The CAT5 jack was extremely corroded and eventually melted and that is when the RDTC failed. Both times the part that failed was the 3.3v regulator on the RDTC board. It was a cheap and easy fix and the board was back up and running both times. If I didn't have the 3.3v regulator on the RDTC board I would have likely taken out the 3.3v regulator on the HM board or the OKI 5V regulator if I was running the RDTC on the +5V line.... So, in event that something on the RDTC board shorts out (due to rain or whatever) the 3.3v regulator kinda acts like a fuse... As a result of that experience I modified the RD3 body so the control box can connect in either direction, so no matter what direction the RD3 sits when connected to your grill the CAT5 jack can face down.