This is unusual but the way I make an access hatch is as follows:
Decide roughly where the hatch is to be and the size Try ro choose an area where there is going to be least foot traffic if possible. This is usually governed by two factors: what you may need to get at and the size of the tiles. I would then screw the remaining substrate at 300mm centres and cut the floorboards slightly more than what was required then screw those seperately at 300mm centres or more depending on what joists were available. Then overply with whatever you have decided to use.
Tile as normal but do not grout. Once dry cut out around
tile or tiles area you want to use as access using a fein machine down through ply. Lift whole section out ensure you can access screws in floor boards then replace board with tiles on. Grout the rest of the floor and Silicon hatch access
I hope there is a good reason for floor access because bath panel access is normally much easier. Good Luck
Kev