Updated Busybox
I'm now running OpenWRT R6 on the router, and I also had to upgrade to the newer Busybox code from Kamikaze (available as a separate package). Apparently the code that requests a DHCP address via the WAN port generates fairly large packets, and a small number of people seem to have problems getting an IP address under certain circumstances because of this, and it looks like RoadRunner's servers are one of those cases. The new Busybox includes code to trim the packs to eliminate extra padding that really served no useful purpose, and I'm now able to pick up an IP address just fine.
That looks to be my last technical hurdle on putting this into production here. All I need to do now are fine tune my firewall rules for port handling, and I should be good to go. Now all I need is some free time to get that worked out... ;)