Solution: Buy routers which you can flash over the vendor firmware with DD-WRT / OpenWRT / Tomato or some other open-source firmware. Although that applies to the router CPU (usually a Broadcom derivative) and not necessarily the radio firmware.
However, sometimes the radio firmware is open-sourced (rarely by the vendor, usually reverse-engineered). If not, even the boxed binary radio firmware can often be tweaked in ways not intended by the vendor, without having to reverse-engineer the whole firmware.
If anyone can solve this for routers that actually have the hardware capability for another channel, it will be the DD-WRT people.