I know this is an old topic but as some people might come across this when searching for ‚somfy‘ (as I did), I figured I’d add my experience.
I control my Somfy shades from Symcon using an RFXcom transceiver. It works fine, I use RfxMgr (the ‚native‘ rfxcom software) to do the programming, and only up/down from Symcon. I wrote my own scripts to initialize and control the rfxcom; it’s not hard but a bit annoying because it’s cumbersome to work with binary data in PHP. I also use the rfxcom for reading cheap temperature and humidity sensors, and cheap PIR sensors, for which misssing a message every now and then is no problem. I can describe how I have it set up if anyone is interested; I haven’t made it into a proper Symcon plugin (yet) though - I don’t see the need yet really.
Also, another way of controlling RTS is by taking an RTS remote and soldering some wires onto the small push buttons inside. I have some Keytis remotes build into wall sockets so that I can control the shades from wall switches that fit with the other switches in the house, and not have to use the ugly Somfy wall switches.