I wonder if you could use one of these....
That is basically a serial interface that broadcasts via bluetooth. You would solder the serial header on the kobo to the Tx/Rx/Vcc/Gnd of the module - then find somewhere to stick it under back cover.
You would then have serial access via bluetooth.
This assumes that the kobo itself puts out Vcc on the serial header. If not we'd need to tap off another point.
You'd also obviously decrease battery life but I suppose you could fit some kind of switch inline on Vcc.... hmmm