Can postfix/pipe run external programs under a random UID?


One of arguments of pipe(8) is "user=" that instructs it to run an
external programunder specified user. For example, the following snippet will
run faxmail(1) under the user faxmail:
fax unix - n n - 1 pipe
flags= user=faxmail argv=/usr/bin/faxmail -d -n ${user}

Is it possible to have postfix select user at random from some list of
preallocated UIDs/usernames? (I'm looking for ways to isolate different
instances of the same command)