I believe Dremel use to make a variable speed controller way back when. My old man has/had one (haven't seen it around, so it could of busted on him).
Anyways, it was simply a box that you plugged your Dremel into and had a speed-control knob. I used to snag it from the old man when i was younger and it came in very handy.
You 'may' (i use that term cautiously) be able to find one still if you look hard enough.