That may be true, but without the implementation of the HA feature there is currently no way for a potential gateway device to join an existing network.
Once HA is implemented that will be possible but that doesn't necessarily mean that you will ever be able to demote a gateway to become a mere router/endnode to circumvent the HA price tier. Hence there are also no APIs (planned) to support "demotion".
The discussion about that is still ongoing.
BTW, what is the reason behind the decision against a partitioned mesh network? Partitioning is allowed and expected to happen any time.
Once network topology APIs will be added your Boron could learn about the fact that it is the only device in its network and your code could call Mesh.off() to dissolve its partition.