I think you are vastly overestimating the willingness and capability of volunteers and underestimating the effort needed to coordinate such an effort.

And having any kind of manual intervention required will almost certainly reduce the reliability of the testing.

This is further complicated by the need to reboot with a different kernel image. Qemu and virtual machines can't do all kinds of hw testing needed.

And in fact, the kernel is already tested like this. Just very irregularly and sporadically. The end users will do the field testing and it is surprisingly effective in finding bugs.