i remember reading somewhere (not here i don't think, though) about a tremolo unit that you could just put on a hardtail guitar with little or no modification..

does this exist, or not? it seems a little improbable, but i'd like to believe i'm not going totally bonkers.

i play noisy hardcore in the vein of coalesce interrupted by lots of isis-esque riffage and ambience, with a healthy dose of screamo dynamics ala funeral diner. it even sounds good sometimes!
There are, actually. None are really 'bolt-on-and-go' though.

You have Bigsby-style vibrato's, that are mounted on your body through a couple of bolts. You'd still have to drill a few major holes in your body though, and they're only good for subtle vibrato's. So no divebombs.

The only one I know of that doesn't require major routing or drilling, is the Stetsbar. They are said to retro-fit almost any stop-tail (so no string through) guitar, but I haven't personally fitted or tried one, so I can't tell you for sure.