I'd just like to update.
I posted this question (actually about drilling f$#^%^ hardened cogs) to a local singlespeeders forum, and someone responded that he had a drilled cog he could send me. It turns out he did that to try the fxed gear thing, and liked it so much he bought a proper hub.
It arrived last Tuesday, and I bolted it up, adjusted tension and chainline, and went for a ride around town. Dang, the thing was a hoot to ride
. I was able to remember to pedal on my own without the bike reminding me. I was a bit choppy when slowing down by putting backpressure on the pedals. Total ride distance was about 5 miles, with a couple of small hills.
On Wednesday, I took it to a local trail system that I ride on a weekly basis (Stony Creek in SE Michigan), for a little dirty play. Hopefully I don't get flamed too badly, but I did ride it on the singletrack. I started with a few miles of doubletrack/fireroads, and then rode a 1 mile stretch of singletrack which is quite flat and basically a beginner/intermediate ride. After that, I rode a few more miles of doubletrack and then on to the park's main singletrack, which has quite a few short climbs, and a very nice flow. I was quite impressed with how I was able to handle the trail on the fixed gear; it seemed quite natural to me. This was helped by the fact that I am very familiar with that trail. I liked the "direct feel" I had with the trail, especially on looser sections.
I was running a front brake (this is my singlespeed mtb), but I only used it a couple of times on the faster downhill (doubletrack) stretches. My legs were sore for a couple of days afterwards, especially my quads. This was in spite of only doing 2 laps of the singletrack, when I usually do 3~4 laps.
Druggies talk about getting another "fix". I think that term is quite fitting: I can feel myself getting hooked after just a couple of fixed gear rides.