I have an Garmin Etrex H that is connected to my Nexus 10 using a USB to Serial converter (Prolific PL2303).
I would be interested in using your app or branching it to make it compatible to older Gamin devices.
There is an Android driver for the PL2303 (seems to be pretty standard). With this app, the Etrex H
can successfully create mock locations:
https://play.google.com/store/apps/details?id=com.agbooth.usbgps