I wrote the nice people at Fireboard and got this response back within an hour:
Alex Hurtado (FireBoard)
Mar 18, 2022, 2:56 PM CDT
Hi James,
Thank you for your question! The battery by itself will not be enough to power the fan for a cook that long, you will need to keep the FireBoard plugged in with the supplied charger. Also, while we know that other fans will work with FireBoard, we do not do any sort of tests with them, just our own drive blower so we cannot verify or confirm any specific functionality, etc.
Thanks,
Alex
FireBoard Labs