Michael, how is this different from just ifconfiging the address from
which you want packets to originate first, and having the address
that's currently the primary one be the alias instead?

i.e., if you're currently doing:

ifconfig ep0 inet 10.0.0.20 netmask 255.255.255.0
ifconfig ep0 inet alias 128.45.71.22 netmask 255.255.255.0

And you want 128.45.71.22 to be the source address, why not just
reverse it:

ifconfig ep0 inet 128.45.71.22 netmask 255.255.255.0
ifconfig ep0 inet alias 10.0.0.20 netmask 255.255.255.0

			       _MelloN_