Sometimes it's appropriate to say "stay focused people", but in this case I think a little "defocusing" is in order.

Erik is heading in this direction already I think but just to kick in my $0.02...

Forget about a noise source to generate perfectly random address/ types/ whatever. No matter what you do there is a statistical probablity that you will get a duplicate. That probability varies depending on the approach but probably never reaches 0.

Instead, plan on the contingency that two slaves will have the same type. Consider how Ethernet is handled and the aforementioned CAN protocol. Arbitration is achieved by virtue of sensing the activity on the comm line prior to transmitting, combined with collision detection. Line monitoring and collision detection is easy with RS485 if you enable the receiver whenever transmitting. This way you can "listen" to what you transmit and give up for a random period of time if collision detected. The lucky slave that gets to transmit successfully gets reassigned by the master to a new address. In this case the random time period may be generated by rand() with a high likelihood of uniqueness.

Well it all sounds good in theory but I'm just suggesting here that the solution lies where you weren't initially looking. Hope this gets you thinking along other avenues.
Best regards,
Steve C.