I was thinking the same thing, but it should not be for just xcsoar.

I am however unsure if the available LXNAV sticks are not suitable to
hook up to some sort of GPIO[1] system (maybe IOIO?). They are not
extremely cheap, but at least they already exist.

Maybe someone owning an LXNAV stick can share a bit of info about how
it connects to the LXNAV devices?