Spice, right now, works on the 'hardware level' in a virtualized environment. You install paravirtualized drivers in Windows or Linux KVM Guest and then they work with the Qemu software to do a very effect remote access. I am guessing that similar things could be possible with just running a special virtual driver on a non-virtualized host. Like how you can have virtual audio cards for networking or bluetooth audio and stuff like that. But just for remote access.
Or you can just do screen capture. There are now, amazingly, very good screen recording software for Linux. You know, for making demos or little youtube videos on how awesome Compiz or whatever. This is quite able to run a 1440x900 15FPS screen capture on very modest hardware without breaking a sweat. Almost no noticable impact. Of course it's using very inefficient, in terms of compression, recording method. A optimized form of WebM or MJPEG or something would probably be good. You can capture individual applications also. What you need then is just some sort of proxy that is capable of sending your keyboard and pointer inputs back to the original application. You should be able to do individual applications that way also.
So there are a few different ways.
I'd really hate to have to have it handled on the toolkit level. I know that it would probably be the cleanest, but I dislike it just because the chances of multiple toolkits being able to do it in a correct and consistant manner is just about nil.