Oh sorry, then I am certainly mistaken. It should be 868 MHz. But it certainly is a different protocol than all the homematic devices, that I use. As far as I can guess it is some on-off-keying protocol without encryption. I think the way to go here would be to find already a low-level implementation that supports fs20 very stable (FHEM often has many devices supported through some hardware, maybe you need to do some digging around there) and then see if this hardware is already supported by any of the plugins here. As far as I know homegear does not support fs20.
I just checked and also saw, that the homegear server now supports MQTT, so maybe this plugin has to be retired anyways for the sake of a good mqtt plugin, which could prove far more stable for the future … so we’ll see.