After thinking about it more, heres my thoughts on the matter (AKA take or leave as much as you want.)
First two thousand words on the subject, and then some more words about those two thousand words:
This drawing is the proof of concept idea I mentioned about having a 5 gallon bucket on the counter. (beginning of previous post)
This drawing is the all out "pimp my graywater" system. (one drain tie-in and one mattress shown for simplicity. Add as many of each as you can for the "super pimpified" system)
Notes regarding 2nd drawing:
A: These elbows are to keep air from getting into the bottom horizontal pipe if the water level in the bucket drops too low. This will keep air out of the mattress(es), maximizing the storage space available. That horizontal pipe is also where you'd tee off to add more drains and/or mattresses
B: Float valve in the bucket turns on city water to keep the system from running dry when the mattress(es) get emptied. The contraption on top of the bucket is just a pipe going above the max water line to act as a vent, and a place to add city water (better to have water pipe at entrance to other pipe: don't have to worry about putting a backflow preventer on the water pipe.
B.2: Not quite sure how the valve would work. probably easiest would be a toilet valve? (then you'd have city water piping inside the bucket, and need a backflow in case it leaked, even if the pipe went back above the water line?) Maybe toilet valve is above max water line on end of pipe, but float at min water line is connected by a rod going through the pipe?
C: Pump inlet at the very bottom of the bucket . If there's more capacity in the bucket below the min water line than in your toilet tank it might be good to have a float switch here to keep the pump from running dry. Otherwise, probably not a big deal.
D: Mattress really needs to be on some sort of platform to allow maximum water storage (otherwise, the pump will be sucking air when the mattress is still 1/2 full). Height really depends on how high you can get your drain diverters: top of highest mattress should be below lowest diverter (which determines max water level). but bottom of lowest mattress should be as high as possible to keep the pump inlet under water.
Material notes:
-For the horizontal pipes I was thinking 2" PVC, probably wouldn't go smaller than 1&1/2", though I'd likely keep the vertical piping the same size as the existing drain line. (and maybe even the whole system)
-Was thinking 5 gallon bucket for pump inlet. Could use almost anything you could fit into crawlspace (though you'd want a couple of gallons between pump inlet and min waterline). If it was tall enough to be above the max waterline, that would be a serious bonus: the only hole you'd need to make would be inlet (A) and you wouldn't have to worry about waterproofing the lid and riser pipe. City water and pump feed could both go over the top edge, and you'd have a lot more room to mess with a toilet valve for the city water. (could cut that 30 gal container to fit)
-I don't see a good way to avoid needing a riser for the mattress(es) since I believe you said the floor is concrete (can't dig down). Wouldn't have to be too high (6"? depends on lowest drain diverter), and could be almost anything: bricks, wood/plywood, build a frame and fill with dirt... just put an old blanket or something over to be sure there's no sharp edges!
-A washer hose would probably be the easiest way to hook up the mattress into the PVC (rather than using PVC the whole way to the mattress; PVC would be sure to keep the mattress valve at the lowest point, but if you had to detach the mattress, you'd have to cut the PVC to unthread the fitting from the mattress)
Well that was longer than I expected! Take into consideration that I do MUCH more brainstorming than putting into practice, so by even doing as much as you've already done you are awesome in my book! Looking forward to seeing the next step of the project, no matter what direction it takes.
Feel free to leave positive comments. Criticism can be e-mailed to b.obama at whitehouse dot gov ..... er, I mean, all questions and comments welcomed!