Writing should be fine as the ECM is executing out of bootmode at that stage and they are pretty stable then, the issue is only with the flaky read routines they have.
I understand many people already have a large base of xcal files to use for flashing so we don't want to disable it but having people brick ECM's because it read bad with EFILive isn't ideal either.

Have you had issues with read/write on the CM871 with the OEM offerings?