Thanks! I just poked around XDA and yes it seems that if the phone has security updates past Dec 2016 then you won't be able to unlock the bootloader. Dirty Santa doesn't work on H910 phones updated past then.
After having a Canadian LG G2 which they abandoned for updates this is a deal breaker. I think I'd rather pay more (or less - Moto G5 Plus?) for something that I can be sure has alternate ROMs available.