Just use plain old TCP, write a client and server, and send some bytes back and forth.
Oh, I thought we deprecated TCP in favor of REST because of Parkinson's Law.
Seriously though, you could even just use UDP for something like this since the payload is more or less constant and is less than a packet in length.
OP says he's already got a web server running though, so his idea isn't really a bad one, since it doesn't really incur any additional overhead to speak of and it can easily be updated dynamically. That being the case, another +1 for libcurl here.
