As soon as I read the title of this thread I knew what you wanted to do, and thought it was a great idea. I'm not TOO worried about the chemical makeup of what I put on it, BUT it certainly calls to exercise a bit of caution in that regard as well....
Space is not a problem as long as they are VERY small drops, hold the phone up to the light, there is room, but far less than it looks like, so again, SMALLLLLLL drops.
You've got the right idea - get REALLY used to the keyboard before you take that permanent step, so you know exactly where the best place for those rivits would be (though frankly, i'd love to have them on EVERY damned key, then i'd have no trouble finding my way around).
It will probably be a month before I mod my phone with this... but I think It's pretty certain I'll try at some point, I use the physical keyboard a lot, never could get the hang of the on screen one.