I agree with Rayna in that the best action is to contact UKVI for current chapter and verse.
It always used to be acceptable to submit an expired passport.
However, since the Immigration Directorate Instructions and Policy Guidance there's just no written information at all.
It might be worth a bit of perseverance with that call to UKVI as I just feel that if the 'old policy' had changed it would have been stated as so. Also, the passport is no longer used for ILR vignette as the BRP is now in place.
Would be great if you could spare some time to update the forum on the current policy when you get info on it.
Incidently, under the old rules you are eligible to apply for ILR 28 days prior to the 24 month anniversary living in UK as a spouse (doesn't include Fiancee time)
It's not always necessarily dependant on FLR expiry date.
Good luck, hope it works out well. I suspect it may.