Hi,

I am used to deploy ADSL PPPv6v4 test-bed with 6WIND routers 
(http://www.6wind.com). Maybe my experience could help you.

> The question is how I get the address(es) for the ethernet(s) without
> manually configuring them, because they're really not going to be stable.

The best way is to use a Prefix Delegation architecture. It is based on 
DHCPv6. Your PPP peer (the BAS) provides you a /48 (or /56) with DHCPv6 over 
your PPPv6 session.
Then, the PD client (the DHCPv6 client) of your CPE sets some /64 prefixes on 
each local ethernet interfaces. These prefixes are advertised by the CPE that 
sends RA for the /64 prefixes that have been set by DHCPv6 on your local 
Ethernet interface.

Regards,
  Vincent