Sorry, you can't. You'll have to call setpriority() in your code. Or you could replace your program with a shell script, f.e.:
% cd ~/pvm3/bin/SUN4 % mv prog prog- % echo 'P=$0"-"; shift; exec nice -10 $P $@' > prog % chmod 755 prog
Then when you spawn prog the shell script will exec prog- (the real thing) at a new priority level.