Have you thought about using an include file? One source I found was http://www.docs.hp.com/en/B2355-90685/ch04s03.html. This would allow you to say:

pageroncall: :include:/usr/local/etc/oncall

in the aliases file and put:

#my turn in the bucket
1234567890@archwireless.net

in the /usr/local/etc/oncall </code> file.
Good Luck