Make a pad connect to two nets with different names in Eagle
Make the jumper be a part, and put it onto the schematic and PCB.

Sparkfun have already made some bridges. I think the one you are describing is called SOLDERJUMPERTRACE in my version of their library.

There are other bridges in SPARKFUN'S Eagle libraries, for example with a very narrow 'neck' of copper, so that it can be connected by default, and cut with a fine knife to remove the connection.

IMHO it seems less hacky to have a part, which encapsulates the behaviour, than arrange some tracks on the PCB that work that way. A proper part shows that it is really intended, and provides some documentation hints for future editors of the ECAD.