Ok, so after reading and thinking about this, the only thing that is really required is the RMD cable, and this got me thinking some more, so I re-read the post I referenced to earlier and this was what I was thinking, and it was mentioned by ttocs:
ttocs wrote:As he said this is nothing more then a volt-meter so why not just use a small wire off of + input to the dist block, and just ground the other side. You can use a relay to make a switched ground to turn off the display when either the deck or key is off. I know it doesn't use the RMD port like you wanted but this would not risk the internals of the block, and still read the voltage at the block. You could get a female jack and then just wire off of that.
So this is what I was looking for. What I will end up doing is using some ring terminals to connect the power and ground wires from underneath the fuse block, and then using a female phone jack end (so I can plug/unplud the RMD into it) on the end of that inside the housing so that I can connect a normal phone jack up to it. Pics are a thousand words...
Then I will just make a new small hole in the case just big enough for the phone wire to come out of so I can disconnect it easily. This combined with the relay should work out best! No soldering required (except for the ring terminals, not necessary, but I will do it anyways

) What do you think?