I have a similar unit to this only 220v version for here in Aus.
I would hesitate to plug this unit into a triac controller as the dremel already has an internal speed regulator.
Even if one were to set this internal speed control to maximum and the regulate via external new unit, bad things might happen.
All a little academic anyhow when one reads through page 3 ( difficult but readable).
http://powertool.manualsonline.com/manuals/mfg/dremel/395.html?p=3
In there it explains how the design of the Dremel series motor is for.......... high speed......... and .........low torque.
In a nutshell, load it up at any low speed and due to it's low torque, it'll more than likely stall or at least slow down to the point where it cooks the guts out of it.
Better to use some form of cheap battery drill I'd imagine.
Or if you want mains connected appliance, a $20 motorised egg whisk or whatever from Kmart with a chuck bolted on one end.
Certainly wouldn't subject my Dremel to such a fate.
Had to initially wait for years to be able to afford one in the first place, and then, don't like to see good gear misused.