What do you have to lose?

1. I'm not positive about the sizes of PVC pipe that are available, but I'm betting you could find one that would work. If not, you can actually BUY commercially manufactured spacers to convert a cassette hub to single speed. For that matter, just leave the rest of the cogs in place.
2. That's what the high and low limit screws on your derailleur are for.

Actually, I think that the biggest peril of this plan is that you might like it. If that's the case, at some point you'll probably have to buy yourself a spiffier, new single speed bike.