[
Cable modem: not sure - does it even get an address? SB5100 has a status
page at 192.168.100.1, but I'm not able to browse that when it is on the
outside edge of the m0n0wall.
]

The modem itself does get an address, but not one that you will ever
see.  It's got a management IP (usually in the 10.x range) that is
hidden by the CMTS system.  When you call support, they can get to the
same status page you see (actually, a more thorough one) by hitting that
management address.

That aside - the fact that you can't browse to the modem's status page
through the m0n0 is suspicious to me, because I have the same modem and
have no problem hitting the status through my soekris box.

Are you 100% sure that you need to be cloning the MAC?  I have Adelphia
and I don't have to.  It's better to avoid it unless you've got explicit
reason for it...