I'm thinking of building a large PVC 'U' trap under my tub. (Maybe 4 to 6 feet tall).
Inside the PVC 'U', I would run a 1/2" flexible copper loop, fed from the cold water source.
The bottom of the 'U' would consist of four 45deg elbows.
I would put a 'T' at the very bottom of the 'U', to use as a clean-out. (Diamond ring trap).
Since greywater is very low pressure, a screw-on cap or PVC valve would work for the clean-out.
The copper entry points would be at the top (using Tees), above the 'U' water line,
so the seals wouldn't have to be super tight. Just water-resistant, like RTV.?.
Don't know how efficient it would be, but it's got to be better than nothing..
I was just thinking about how these things work. In addition to recycling
the pre-heated water, some of your lost body heat would also be recycled..