Hmmm,

That might be a better solution than epoxy - expose the ground layer next to the UCB plug and run a bead of solder onto it and then the plug.

Or alternatively if you still want to do the epoxy it would not hurt to take a nice sharp knife and scratch the ground plane below where you plan to put your epoxy - this provides for a nice mechanical grip for the epoxy to hold onto.

On the subject of epoxy I have also thought of a good way to ensure that any epoxy that might run into the plugs holes does effect the usb plug. Borrowing on what someone else posted about spraying silicon on your usb plug - do this and then carefully insert the usb plug into the socket before applying epoxy. Leave the plug in during the full epoxy drying time and then remove. The idea being that with silicon on it the plug will not adhere to any stray epoxy that might get through the holes, and, since no two bodies can occupy the same space in time, the plug provided a mold to ensure that any stray epoxy does not affect further insertion of the USB plug.