I see two questions. First; can we infer from a process's niceness or scheduler class whether it would prefer waiting for a hugepage or taking what's available now? Second; are memory compaction passes preemptible? Is this the behavior you're looking for?
1. A low-priority, sched_idle process (1) tries to allocate memory. The kernel starts compacting memory to provide it with a hugepage.
2. A higher-priority, sched_fifo process (2) becomes runnable and tries to allocate. Because it's higher priority, the kernel puts the request for (1) on the back burner. Because (2) is sched_fifo, the kernel doesn't wait for compaction but just gives it what's available now
3. With that request satisfied, the kernel goes back to compacting in order to satisfy (1)'s needs.
As someone who only uses -rt kernels, this is the behavior I think I would want. The network can get hugepages, and it can wait for them; but jackd and friends better get absolute preferential treatment for memory Right Now.