Certainly in some areas councils will allow usage prior to 0930 - particularly on services they are subsidising anyway. I can get to my nearest large town on a bus leaving before 0900 despite it being an hourly service
And the services from Heathrow out to Slough/Maidenhead allow it - certainly beyond the normal free bus limit if not the whole route
But there is always a problem with cut offs - fine tuning what they are rather than discretion is an answer - though maybe a bureaucratic nightmare
If a passenger has a Freedom Pass this is valid on the First services between Heathrow and the London boundary at all times in line with validity on TfL-sponsored services. Slough council also allows use before 0930 for its pass holder. Strictly speaking other ENCTS passes are only valid from 0930 but it wouldn't surprise me if they were accepted as there'll be very few of them.