Of course, ceiling variance isn't of much concern if you're doing an outdoors-y stage. Also, you'll get more actual LEVEL done if you ignore FOFs - it'll limit what you can do, but there still IS a lot that can be done without them.
My only concern would be how easy it would be to do with SRB2, period; I know it can be done with DOOM, since it WAS done with DOOM, and done very well. But, those were the traditional indoors-y stages, for the most part, which SRB2 doesn't always excel at...
Oh, and I suppose Thok Barriers don't count, except for any EXTRA ones you add for effect (like a change in Floor height, for instance).