Record with SimpleScreenRecorder to one Workspace only

I don't know about that, but perhaps you'd be interested in Open Broadcaster Software (OBS)? This program lets you record windows directly, so it might do what you want.

The one thing about OBS is the official binaries depend on libfdk-aac, which is in Ubuntu's non-free repository and therefore not in Trisquel, but I tend to think this is a bad judgment on someone's part; its license is GPL-incompatible, but seems to me to be libre. (If you think this is wrong, though, I suspect you can remove the dependency just by compiling yourself.)