ilikemycat wrote "First off, I think flat field lenses like enlarger lenses would be better than extension tubes and macro lenses"
Stuff and nonsense! There are exceptions, but few enlarging lenses will beat Canon's own macro lenses on 35 mm. I'm a Nikonist, use 55/2.8 (great), 105/2.8 (great), and 200/4 IF (middling) AIS MicroNIkkors. I also shoot closeup on 2x3, my preferred lenses there are a 100/6.3 Reichert Neupolar (better by test than a known good 100/6.3 Luminar) and (here's the exception) a 4"/5.6 Enlarging Pro Raptar. My 105/2.8 MicroNikkor isn't quite up to these marvels, and my 55/2.8 MicroNikkor isn't quite up to a 40/4.5 Luminar at magnifications above 1:1, but for out and about use the MicroNikkors are (a) much much better than good enough and (b) so much easier to use that I've never seriously considered using my best macro lenses on my Nikons. Same goes for Canon's answers to my MicroNikkors.
Questions of attainable image quality aside, the big problem with using a lens with manual diaphragm such as an enlarging lens or any of my really good macro lenses on a 35 mm SLR is stopping down to take the shot. Auto diaphragm with viewing at full aperture makes shooting closeup much easier. People who know only modern SLRs don't appreciate how good they have it.
mwelsh, if you don't have it, buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Also buy a copy of A. A. Blaker's book Field Photography. Both books make the point that poor technique always beats good equipment and explain good closeup techique in detail. Field Photography is a little better for beginners and near-beginners who have 35 mm SLRs.
You should also learn how to use flash closeup. Flash on a 35 mm SLR greatly reduces the difficulty of getting good results and allows shooting handheld. Blaker explains it well.