@kersing already gave the perfect answer to this with respect to the EU band.
Also for North America, I don't see this working. You're right that eventually a join woud get through, but a node is required to operate at more than one frequency, even in hybrid mode.
The single_channel_gateway is great to experiment, but I'd rather not see them as part of the TTN deployment. The prices of real gateways are not out of reach anymore, and with those we can build a scalable system. Let's not break it before we start 