would an AFI work, if you had a back up power (5kw generator) feeding the breaker?

The breaker would be fed through a manual transfer switch, one side of the switch from
the AFI and the other from the generator.

It seems like the breaker would be an intergal part of the neutral since the banch circuit is landed in it and then bonded to the nutral busbar. And if the breaker is de-energized would it be an open circuit when the generator tries to feed it?

Any thoughts would be appreciated.
Thanks.

Use a double pole double throw transfer switch and it will be a non issue. If you transfer both the neutral and the hot the generator functions as a derived system. The easiest way to provide grounding will depend on how your generator is configured. If its neutral is bonded to the frame then you will need to run a Grounding Electrode Conductor (GEC) from your house's grounding electrode system to the bonding lug on your generator. If the generator neutral is not bonded to the generator frame then it will be easier to bond the neutral to ground in the transfer switch and run the GEC to the grounding electrode system from the switch's enclosure.
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Tom H

How do you intend to comply with 210.12 during the time the generator is supplying power?

well 702.3 is specific.
the only thing left is to get a blessing from the "ahj" and filling out a proposal to the panel for an exception here.