I want to dress up the muzzle end of my A2 barreled shorty carbine, but I've no idea what options are avaliable. I'd like something that would may be slip over the barrel and give the appearance of a long flash-hider on a shorter barrel- anyone know if these are produced for post-ban guns? (Something in say, a set screw device?)

There are plenty of set screw options out there.  One thing you may want to look at is sending your upper to Cavalry Arms (click on their banner or go to Industry Forums) and having Shawn install one of his "cav comps".  They are a muzzle brake that closely resembles an A2 flashider, plus you don't have to worry about the set screws coming loose and shooting the brake off the front of your gun.  That actually happened to me, not fun.  Kurts Kustom Firearms has something similar as well.  Look into it.

Ratters, thanks a bunch for the suggestions.

I've been tossing around the idea of putting a segmented sleeve on the barrel of cheap pipe or PVC to make it resemble an M-4, and then have one of those breaks attached perminantly.

This is my first AR, I've carried 'em in the guard, and shot them in ROTC before that, and was never overly impressed until I got my hands on an M-4 at a recent field excercise. Before that I'd been extensively into AKs- in those rifles I have seen many 'shoot-offs' of muzzle breaks. The most humerous being my best friend in highschool who got a pin-on break for his post ban. He had to tap it on with a mallet and figured that friction would do it. A hot day, a few rounds, and a messed up muzzle.

There are also some fake flash hiders out there if you are just going for looks.  Sorry, don't have any links right now.  I avoid brakes because of the increased noise.

You might want to check out the brake on this link.  Scroll down till you see the muzzel brake. It has 4 set screws

http://www.olyarms.com/bblret.html

From the picture looks pretty good.
(didn't come out a clickable link but the ol' copy and past thing will work)

I had one of those Olympic brakes on my frakengun, I wasn't very happy with it. When I first took it to the range I tightened all four set screws, fired about 200 rounds through it over the course of an hour, then put it away. When I got home and started cleaning it I noticed the the brake was loose, so I tried taking it off, but it wouldn't come off with all four of the set screws removed. The set screws had come loose while I was firing it and allowed the brake to slap into the ring machined into the barrel. It took me about 20 minutes with a Dremel tool, grinding through the holes where the set screws were to get it to come off. Now I have a barrel with four lovely gouges around the front of it.

Next time I put a break on one of my rifles it is definitely going to be one of the screwed/soldered on type.