How much programming are you willing to do to get a static process
priority?  All you need to do is suppress the line that says:

	p->p_estcpu = ESTCPULIM(p->p_estcpu + 1);

in kern/kern_synch.c.  You might do this based on a bit you stick
in struct proc, and you might want to set that bit from within
sys_setpriority(). Define a magic argument value or some other
quick hack that you can easily recognize with a line or two added
in sys_setpriority().

After you get a chance to try it out, and if it works nicely for you,
then you could think about formalizing the implementation a bit.

//ross