I see you already answered you question, but a few comments anyway...
I had the same problem with speed/torque. I was trying to use a Type 27 flap wheel to remove old paint. If you spin the wheel fast (normal grinder speed) the paint heats up and sticks to the flap wheel. Slowing the wheel down helps, but still isn't practical except for small areas. I would adjust the speed loaded, which is considerably faster unloaded.
The other problem is that by slowing the motor down you reduce the airflow cooling it. I was using a Harbor Freight Grinder, the paddle switch model so it wouldn't be a terrible loss. But I didn't have any problem with that and this was during summerish weather here. Overheating was in the back of my mind though and I tried not to push it too hard...
To get this to work you would need a feedback circuit. Something that measured the output rpm and then controlled the juice to the motor. But I'm sure you've already figured that out ;-)
Not sure why you are looking to reduce speed. In my case I wanted to use Scotchbrite pads for surface prep and these burned at the full 11,000rpm. In the end I got a variable speed Makita 5" grinder which works just fine. Is that an option for you?
It's likely that the HF router speed control is a very simple diac/triac circuit. These are very useful for lighting loads, lowering the temperature of a soldering iron, or other small loads (the label rating suggests 15A max, IIRC).
The trigger/switch circuit in VSR variable speed drills (for example) is a better circuit for maintaining motor torque at lower speeds. This is a characteristic of PWM pulse width modulated circuits. Many PWM circuits don't require a speed sensor device at the motor, so they can operate well with only 2 wires for motor power.
I used a variac recently on my 4.5" grinder to slow it down enough to use a 9" abrasive wheel. This was done to give me access to the inside of a welded C channel which was too deep to use with 4.5" wheels.
It took a couple of goes to dial in the right voltage to give me the proper speed under load, but for my limited need it worked fine.
I don't think my grinder liked the additional mass of the 9" wheel, though, but for a quick grind it worked fine.
They do have pneumatic versions, should you have shop air. These tend to be smaller and lighter for the same work capacity and readily throttled. They had air routers at the motor home plant for doing cutouts that had throttles on them, seemed to have plenty of power. Pneumatics don't have the heat buildup problems that electric motors do, just need good lubrication.
You can run series wound universals using DC servo amps too, as long as the amp has a setting to run in IR mode--I've done this several times and generally you'll get fairly stablw speed control under widely varying loads.
Specifically, I used the AMC model 25A20--one thing though, it's quite possible to overspeed the motor with this arrangement in which case the results could be downright spectacular.