I am only going by what I have read so far, the mac address for your hacked modem looks like it is decided by the config that you use for the modem. The config file that you use must be similar to yours but not the same i.e the config file must be for your area but not your 'ubr' otherwise you will knock each other off the network. I fyou think you have got your modem all set up than have look in the mac trade thread and see if someone can supply you with a mac address.
The ubr is like a gateway and so as long as both of you are not on the same gateway you will be fine. I would give it a go, try it without your router first, no point in having unnecessary complications.
just select your adaptor and hit the random buton whenever you use your dodgy modem, when/if you use your legit modem hit the remove button. is the modem going to be connected to a router?