Just spit ballin' here:
Given that the Nest Protects are hardwired (yes, I'm ignoring the pure battery ones) could they run ethernet over powerline in addition to the smoke alarm to smoke alarm communication. Then the Nest Protect with the strongest wireless signal would bridge with that network and the other Protects would be come wireless access points.

Having said all that, it would be a shift away from the core product offering, and likely increase the number of support calls, for minimal gain in revenue.