I was looking at the GoPro LCD that you can buy for 70 dollars and started to think that it might be possible (if I figured out what kind of port connects to the GoPro) to adapt it via an HDMI is there is a connection to do so.
First of all i can tell you that the LCD from a go pro has 2 problems. 1 its got a proprietary connector so you will need to decode that wiring to figure out what to put where. and 2nd its resolution is pretty low. For framing you would be fine but focus and exposure wouldn't be useable on this display.
For the cheapo little 3" lcd from cheesycam, same problem with the resolution, low but useable for framing. If you have focus peaking on your camera then its OK for focus but not great. exposure is iffy because you don't know how accurate it is. Wiring and power is easy though!