The only issue I can see is the line level inputs on the OT. You'd need a preamp, too, most-likely.
Myself, I use a Zoom H2n (field recorder) since I already own it, it's pretty small, and has a 20 hour battery life. And a headphone out I can plug into the OT with a 3.5mm (1/8") to 6.3mm (1/4")
Or a smartphone if you don't mind the lo-fo mic feel
Though I suppose you could indeed construct a low-power preamp and mic into one little enclosure, nothing fancy.