Nick,
Actually we agree on the calerpa.
I dont like it as a nutrient transport either. I just mentioned it as an alternative. I use a large turf scrubber, which uses turf algae. I once tried the large sump/refugium/calerpa thing. It may work for many, but not me.
Please excuse my reference to an old tank of mine on TOTM, but its the only pics I have left of that system.
http://reefcentral.com/totm/2001-04
As to your idea for a bottom drain. I have wondered about something like that also. Would it not be similar to those that have made a drain that runs up to the top of the tank for sampling? I think Charles D. was doing some tests along that like at the Waikiki Aquarium, {not sure}, but have heard no results.
I know some have used it as sort of a slow flow undergravel, to change out some plenum water. My worries about that would be escaping nitrates or something.
Also wonder how much the massive draining from a bottom valve, would effect the low oxygen levels, desired in the plenum itself?