I'm very tempted to do a Protozoic (Carboniferous or Permian) mod for DF. Probably Permian, unless you want only ferns for trees and no way to brew anything (as far as I know, there aren't any mosses or ferns that are fermentable).
Edit: Whoops, I was wrong. The Carboniferous and Permian seem to have been dominated by seed-bearing plants like the erroneously named "seed ferns" (actually early gymnosperms, not pteridophytes). The real question then is whether any of these seed-bearing plants had enough sugars to allow fermentation- the later Permian did have conifers and ginkgos. You can't do mead instead, since there are no flowering plants and thus no bees. No mammals- no milk or cheese, unless (as I think there might have been) there were some mammal-like reptiles that did nurse their young, without being strict mammals- but that'd be late Permian. Since most coal deposits didn't even begin to form until the Carboniferous, and none earlier (to my knowledge), I don't think there'd be any such thing as decent coal deposits- all smelting would have to be done via wood charcoal. Flux stone could also present a problem. There'd be some limestone, but chalk doesn't form until the Cretaceous (hence the name). I'm not sure about dolomite (Wikipedia simply says "vast deposits exist in the geologic record"), and marble is simply metamorphosised dolomite or calcite- so I'm also not sure about marble. You'd still be able to get eggs from the reptiles, but there aren't any birds. So you might just be able to do late Permian as a mod. I think I might work on that...