yep, i've done it. if you're handy with a soldering iron, it's not that tough.
some general things:
the standard Crybaby-style case has two knock-outs on either side of the switch; if drilled out, they can hold a standard 5mm LED in a plastic mounting bezel. or you can drill/mount elsewhere.
be careful when drilling! so you don't scratch up the casing or punch through into something else.
you'll need to figure out what value of resistor to use as the current-limiting resistor that limits how much current goes to the LED. it depends on what type & brightness of LED you get. online links will tell you how to calculate the resistor value; i did mine mostly by trial & error testing before i installed.
to avoid pops when turning on the pedal, the best place on the board to tap the power from is the filtering circuitry used for the power-adapter jack. if your RMC doesn't have the wiring for a power adapter, you may just have to tap off the 9v battery lines.
pick a cool color LED.
i got the parts for mine from Small Bear. i have seen kits on Ebay, but i don't know if those guys know their stuff well or not.
my fav online reference is
http://stinkfoot.se/archives/553. there's tons of others; google and ye shall find.
here's pics of one of mine (also has true-bypass