I was at the habitat for humanity store the other day, and they had a pair of those "wireless phone line" extenders for $2.50. I picked them up and brought them home, thinking I could fake it into sending serial data, ethernet, or something like that.
And then I thought about it-- I'm pretty sure they're "wireless" in that you don't have to string wire between them. Also pretty sure they inject the signal into the house wiring. I don't know that this unit is one of those for sure, but the size of the transformer and the lack of any clear aerial makes me think it is. I'll be playing "hunt the datasheet" later on to see if I'm right.
If it is, I wonder if it'd be possible to cut out the POTS stuff and use it as an injector for carrier current AM?