Cool, I've been thinking of alternate housings also.
First you need to have your MB manual or at least a diagram of pinouts, gotta get the right pins. Sometimes you can read some markings(not me, I have to use a magnifying glass all the time now
) on the MB board.
Most peeps use, like you say, a momentary switch(i.e normally open, electronic term.). Note, some older cases are wired diff. for power(i.e. double pull/throw switch).
It helps if you have the connector(i.e. 2-pin). I used a screw driver to short the two pins together(not recommend!) temp. for that build, because I couldn't remove the existing case switch w/out damaging it.
You could use a on/off switch and just toggle it on then off right away to get that momentary effect, but that's not really recommended.
I do find it hard to find an appropriate momentary switch for the PC, but a rocker type from Rshack would most likely work fine. The trick is to find a mounting style that works well for your installation.