Linux isolated through virtualization under a microkernel is the way to go if running Linux in the same hardware is actually needed at all. The realtime tasks would simply run outside of Linux, shielded from Linux.

Genode's virtualization (particularly when paired with the excellent seL4 microkernel) is a good way to achieve that.

If Linux can interfere with the "hard realtime" task, then there can be no assurance (of meeting hard deadlines) and it's thus not a solution providing hard realtime.