There is a tutorial on LL:http://www.luminous-landscape.com/tutorials/understanding-series/mirror-lock.shtml
that describes a small test - putting a small container of water on the hot shoe.
I tried it a couple of weeks ago - with some surprises.
I set my tripod up on the living room floor (standard nominal 2 x 10 joists with 3/4" sheathing and 1 1/2" concrete topping) with a 100 macro and a dish of water on top of the hot shoe. I used a remote/cable shutter release, and set the exposure time to ten seconds.
1) the mirror-up slap was very obvious - ripples immediately,
2) when the ripples stopped (this took just over one second), I pressed the remote button again to open the shutter,
3) surprisingly from the shutter opening, there were more ripples, although noticeably less than from the mirror slap.
4) at the end of the 10 second exposure, the shutter closed resulting in more ripples - this of course has no effect on the image.
5) what I did notice that bothered me was that I had to keep the cord attached to the camera perfectly still or the very small force from the moving release cable would cause ripples.