Yes it is very possible.
Actualy, I've had a small portable Philips TV with a hack like that as a monitor for my Atari ST for years.
My dad had a "videodata" (sort of prequel of internet) terminal in the office. This used a keyboard that was hooked up to a PCB that was completely mounted inside this portable TV. There were a number of connectors on it, for the modem, printer etc.
Anyway, since this terminal was obsolete, and I couldn't afford a real color monitor then, AND judging by the picture quality (excellent) I figured that the PCB/computer inside was sending the "monitor" RGB. I contacted the company and asked if they were interested in a spare PCB and keyboard and indeed they were. I asked about the RGB and they confirmed. I told them they could have the PCB and keyboard if they'd mount a SCART connector with RGB for me, and they agreed (I admit I was amazed at the time)....
{Edit}
Thinking about it, you could probably feed just the composite sync to a composite video input instead...if that's on the TV set.
It worked pretty damn good for years....I really wonder what I did with it. Throw it away or is it still somewhere in my attic ? If so, it will be a great little monitor for testing.
I guess the hack is fairly easy if you got the schematics. Almost every relatively recent TV has some decoder chip that does almost all the work for the color decoding. If you find the pins that put out the decoded RGB, you can probably simply lift the legs and put in the RGB signals.
The trickier part is supplying the sync signals, but the same applies here, if you can read the schematics and find a proper place to connect the external sync signals, it will definitely work. You gotta know what you're doing however ! For all the above, the levels (voltages) of the mentioned signals should be (about) the same as what the schematic expect at that point of entry.