To the best of my knowledge, no. Your best bet would be to create a shell script which runs your executable using nice. In the case of application packages, you can simply move the executable aside and put in its place that shell script. Here's what I used for Mail:
I had moved Mail to Mail.bak, saved this shell script as Mail, then ran 'ps -xO nice' to verify my success (irrelevant lines removed):
nice -n +5 $0.bak $1
I can't guarantee that this will work for all programs, as some finicky ones (*cough* STARCRAFT *cough*) seem not to obey all kinds of guidelines. Note that I couldn't use ./Mail.bak because applications launched by the Finder (Launch Services actually) have their working dir at /. Also note that I included $1, to preserve LS's -psn_X_XXXXXXX argument. Someday I'll know what it really means.
PID NI TT STAT TIME COMMAND
1180 5 ?? SN 0:06.05 /Applications/Mail.app/Contents/MacOS/Mail.bak -psn_0_9043969