hey, I think you can do this - I tried it out on a spare non powered mixer, but you have to run things around in a funny way - there is a diagram in an article floating around here about making 'lo-fi' funk music - its called "Shitty is Pretty" (the author might have been Gabriel Roth?), there's a diagram that shows you where to plug everything in. I tried it and it worked sort of, but i got a better effect using a headphone amplifier to power the spring....
so, If you can find a headphone amplifier i suggest:
-effect send out (from mixer) > headphone amp > input on spring tank : output on spring tank > mic preamp on mixer.
of course you'll want to be sure not to have any effect send on the channel you're bringing the verb tank signal from to avoid feedback (though if your careful i guess you could play with that a little for 'special effects')
sorry i don't have that 'shitty is pretty' article handy, but if you do a search i guess you'll find it.
let us know how it goes!