Thanks for all the great replies. Answered all my questions and then some.

I ended up stopping by bfold and talked to Steve. Here's the saddle he uses.
http://www.ismseat.com/products_sport.htm

for my part, I ended up putting my brooks b67 on my brompton. This is because I ended up deciding to get a crummy old hybrid bike on CL to use as my daily workhorse ride, since I didn't like the feel of a tag-along trailer with the folder. So my brommie will be spending less time on the street. Now I gotta decide on an ergo seat for the old hybrid though...