“can stand up to water, mud, dirt within reason, assuming I clean it when I return home. (it's nature photography, so..duh) It is possible that in some cases I might be actually submerging the legs in water to a decent height (i.e. more than just a few inches)”
Some time ago, I found a set of “tripod booties” on the Internet. These were plastic tubes which were sealed on one end. Placing the tripod legs in the booties protected them against water and sand. I don’t remember how they were kept on the tripod legs and I cannot find them again....
“I will be using a ballhead for nature in general, and a gimbal head for wildlife”
I use a Manfrotto gimbal bracket for long lenses on both tripods and monopods...
"I probably don't need (and would prefer not to have) a center column at all. Or have the ability to replace the center column with an extremely short one, or take a hacksaw to it, etc”
Some tripod manufacturers offer shorter center columns. However other tripods, like some Giottos models, articulate and make very good use of the center column to place the camera exactly where you want it.
http://www.giottos.com/MTL-9.htm
These are not the booties that I originally found but, it may give you some DIY ideas...
http://www.nickcarverphotography.com/blog/?p=2243