[Hugh Jim Bissel]

A waterbed mattress sounds like a very good idea! Also, seems to me overflow wouldn't even need to be an issue: have a hose/pipe running from the mattress up to a container on the countertop: when the mattress has room, the water goes from the washer hose into the container, down another hose/pipe into the mattress. If the mattress is full, the water level will be up in the container, so additional water flows out the container's overflow into the utility sink.

Then, if you slant the mattress (or dig down a touch at the drain point) so the drain/fill is at the lowest point, and tee the two hoses (to container by sink and to pump) there, you won't get any air in the system unless you're almost completely empty. (how much room do you have? you could fill your whole crawlspace with mattresses!)

Sounds like a theory, anyway!

edit: If you want to get really ambitious, since you'd be storing water in the crawlspace, you could put that sort of diverter on any drain you wanted. (I assume a: you have access to your drain lines in the crawlspace, and b: they are pvc or something easily modified vs cast iron or the like) It wouldn't even need to be a container: just a tee of some sort where the water drains into the waterbed leg unless that leg is full of water: then it has an out to the regular drain (ie sideways tee: source at top, storage at bottom, and drain at side)

edit2: if you went that direction, you wouldn't even need the container by the sink. just mod the washer drain, or mod the sink drain and put the washer hose in the sink.

edit the third: And once you've got your crawlspace filled with water coming from all your drains, don't forget to tee off after the pump and put in a hose bib so you can water your garden & lawn with all your free water! (since your objective is to keep it out of the sewage tank)