I wonder if one of those Ebay flat plate exchanger would work for a shower drain (warming the cold input side).
50 Plate Outdoor Wood Furnace Heat Exchanger 1-1/4"Port - eBay (item 250586847892 end time Mar-26-10 22:34:19 PDT)
Might be a little tight in there..
Have to build in a back-flush valve to clean it out..