I think the biggest problem here is that most (if not all) of those OS's are not finished. Someone would have to create a whole new image every time an update was released for one of them.

You'd also have to have an eMMC image as well as rootfs so that would restrict peoples freedom on partition sizes and install locations and means smaller MyDocs for everyone.

I also dont know if all these operating systems can be installed on eMMC so you might need a image for SD cards instead.