It turns out that you can already buy customizable wireless router platforms (called Wirless Router Application Platform.. started by a company called PC Engines). For about $150, you get a mini board with a 266Mhz Geode processor builtin in, 128mb of sd ram, a CF slot, a couple ethernet ports, and a couple mini-pci slots (in which you can stick in linux-supported atheros-based wifi cards). So for about $200-250 and a little bit of work, you can get your very own customizable router.
My only worry is whether the 266mhz geode is fast enough for 100mbit of NAT.