rpc has ZERO security. Either you care about security that works over the internet or you don't. If you can guaranty that connections from the internet will always be through a secure tunnel or VPN, then use whatever lightweight communication method you want.
I'm honestly surprised that you believe ssh is too complex. It is much simpler than running a web server that handles any sort of request - perhaps 100x easier.
Have you never run a remote program from an ssh request?
The returned data is displayed on the local server. That means it could easily be captured and used just alike any JSON you like. Complicated?
$ ssh user@server ls /bin/
The requests can be made with an XML or JSON input too. Very simple, very "RESTful", very secure.
$ ssh user@server my-fancy-server optionA optionB .... optionZ