First off, I would NOT recommend trying this.....
That said, a guy I knew had a combination mill/metal lathe that had mechanical, but limited, speed control, with an induction motor. He put one of the router speed controls that you can get cheap at Harbor Freight. He makes sure it is set a full speed, turns the machine on, and proceeds to slow the motor with it. I was amazed he did that with out smoking the motor!
So, while it TECHNICALLY worked, he is very likely adding a LOT of extra heat to the motor's windings. I can't imagine that will continue to work as a long term solution for him, and if I were to guess, he will end up with a bad motor of continued use with it. Not to mention the drastic decrease in working power of the machine.
So again, I would NOT recommending putting a speed control on a universal motor, as you risk permanently destroying it.