*snip* it seems as though I can simply leave the NAT functions on the bridge/repeater turned on, and this will give me the network separation I'm looking for.
Keep a watch out when you get to that point. I'm not so sure that any traffic will be crossing the WAN/LAN boundry. It is hard to tell without seeing it first hand but from the wiki it reads like the ddwrt firmware is creating a LAN/LAN repeater boundry and the WAN interface remains unused. Maybe the firmware in repeater mode re-purposes the WAN interface as the second wireless interface.
The mention in the wiki of
Open the Set-up -> Advanced Routing tab and change the mode to "router" instead of "gateway".
will be interesting to co-exist with the prior instructions to use the same IP schema as the primary router. I don't see how it could route properly without having a different schema much less avoid dhcp collisions in that scenario. The wiki is confusing so I may be confused about which interface is being setup at various points.
The wiki also mentions having the bridge/repeater use the primary router for DNS. In practice DNS relays are flaky on a lot of firmware in soho routers. Watch out for that too. Use the actual DNS addresses provided by the ISP in both routers if DNS relay is flaky.
Regarding the 50% bandwidth. Keep in mind, its not just about the size of the pipe. Its also about packet loss and delay. Test for latency and consistency of connectivity as well before relying on it for streaming.
Post an update once you get things going. I'm curious how the setup will perform.