Noting that's just a setting in the firmware on the ONT. If you think that can't be changed on a third party ONT to match the Openreach ONT (and the MAC address for good measure), then I have a slightly used bridge to sell you with only a few snapped cables and brand new truss end links 
Indeed - however whilst the basic G-PON functions are standard there are a lot of other bits around it that are used to manage/control FTTH networks that are not, so Openreach/other PON providers are very likely to spot it is not one of their ONTs (if they care). A lot of the service monitoring/support data you can get off ONTs is either not consistency implemented or is proprietary.
Openreach support Ethernet OAM for example, would a third party one work the same way?
There is a lot of work ongoing with some vendors to certify third party ONTs to give more flexibility for different deployment scenarios. ONTs on an SFP are an interesting one, the end-device just sees a standard SFP, so it means you can take a lot of sensors and connect them to PON, lots of potential for Smart Cities and small 5G cells.