I know this is an old thread but I thought I wold throw some info on this. I came across an RT that would not boot up (ie. screen was black). Checked the charger output... it was putting out 12v as expected and the charge light was lit. Anyway, as the OP indicated, I decided to crack open the RT. I check the battery at the cable. It was in and around 8v or so and was charged in some form. I pushed on the power button and saw the same red light come on. It stayed solid lit. In fact, I could leave it in that mode for hours and it stayed lit.... but I could see after a number of hours, that the battery voltage (as tested with a meter) was going down to the low 7v... so it appears that the MB was trying to do something. Trying some of the reset operations (ie. holding the power button down for 10 seconds, etc) resulted in the light going off. So basically, the light appeared to be some form of MB status light.
Anyway, I did some measurements of a few things that can be found without a schematic and knowledge of the RT "guts". The one area in particular was the 5v pins on the USB and the dock / keyboard connector. In both, there was never any voltage there whether the unit was plugged in with the adapter or "running" (ie. red light lit) on battery. Without much further knowledge of the design, I purchased a used MB that was operationally reset by the previous owner. Removed the old board, and installed the replacement. connected the battery.... the red light lit solid for about 4-5 seconds and then started flashing (better than the old MB) and after about 5-10 seconds or so the screen lit up and the tablet was alive... yes!
So if the red light on the motherboard does not ever start to flash after trying to start up, then it is likely that your MB has a significant problem. Probably most user would not even attempt to open their unit or replace parts.... but with the many internet sites and info , these types of things (especially out of warranty and old) can readily be done by some users (or friends of users).
Hope this help save an occasional RT.