That's a good point. But I talked about RTAI or RTLinux because the Xenomai reference. My experience is with RTLinux internals and the virtualization made there is just the simplest one, so you do not have isolation. You know there are today solutions based on full virtualization even with hardware help which, I guess, is a must for real full isolation.
The good part of RTLinux technique (I think RTAI did not follow the rule of "keep simple" strictly) is you can use it with some microcontrollers which will not have support for hardware virtualization.
Don't take me wrong. I think you are doing a fantastic job and with your mindset, maybe you prove me wrong in the future. I just wanted to point out the challenge (probably) needs another revolution regarding kernel design. By the way, the kernel modularity could be the starting point.