Anyone running whatever they want on RouterOS sounds like support nightmare.
But perhaps if there was at least some limited environment, some kind of chroot, limited access to selected parts of hardware, non-root user with limited capabilities exposed, etc ... then it might be feasible? Or in worst case an ability to activate some unsupported mode, i.e. "we let you do it, but if you do, don't ask for help with anything, including the rest of system". Not ideal, but still better than nothing.
There's MetaRouter, where it's possible to do a lot of stuff, but it's the matter of resources. A lot of things can be done using few kilobytes of disk space and memory, but when you have to wrap it in another whole OS for MR, it makes huge difference.
And if there was an interface for these custom packages to expose themselves in ROS console or even in nice and friendly WinBox GUI, imagine the possibilities. I see it already, things like original full-featured OpenVPN, various servers, all crazy dreams could come true.