Hmm. A Tannerin with a volume loop. That tickles me. :-) Theremin's earliest instruments had a pitch rod and a manual volume control. I like seeing ideas turned on their head!
As a theremin player who found his own way of side-stepping the pitch stumbling block, (I play non-pitch-critical music only) I find it an interesting notion. Rather than, as Christopher sees it, a step down from a theremin, I see it as step up from a tannerin. Just as a theremin is a more expressive instrument with a volume loop (aka expression loop) than with a volume pedal, slider or knob so might a tannerin be likewise.
Volume-only theremins are a rare beast indeed. Z-Vex have discontinued their volume probe (http://zvex.com/probe.html) so it might be worth asking Zach about a schematic for your own personal use - no redistribution, no commercial interest (my opinion is, "if you don't ask you don't get" and, "the worst they can say is No") or you could an eye on eBay for one. I also have seen a theremin that comes in two modules - with the expectation that you would buy the pitch module first - somewhere on myspace - I don't recall where, and having met someone who bought one and was disappointed I am not in a rush to find it again. One could adapt the volume circuit from a pitch-and-volume theremin, or perhaps Andrey Smirnov's theremin sensor (http://asmir.theremin.ru/tsensors_sch.htm) #3 could be useful here.
Also - Rich, have you seen Theremin's take on the fingerboard - the Theremin Cello (http://www.peterpringle.com/cello.html) - from a performance point of view that's got to look better than a tannerin. How would you feel about a slide-keytar!