I dug into it, and it looks like the PRI macro should work. See news releases below.
Try a template like this:
destination self { udp("
cashmere.comp.uvic.ca" template("<$PRI>$FACILITY $PRIORITY From $HOST: $MESSAGE\n"));
News for the 1.6.3 release
Thu, 06 May 2004 11:05:46 +0200
* fixed afunix and afinet destination template handling, do not include
the PRI value automatically, let the administrator do it
explicitly in its template
News for the 1.6.1 release
Fri, 9 Jan 2004 10:27:38 +0100
* added a new macro named "PRI" which inserts the decimal value of
the message priority (pri + level value combined)