Thanks for any input!

Basement level egress windows in my neighborhood have grates over the well, and for security reasons they have a chain that attaches to a hasp with an easily removable pin, from inside the window well.

It keeps people from getting in through your window but allows you to easily open it from the inside. You would not need it to lock down like this but the grate of this type would work. I would think that a wooden slatted "grate" that fit into a recess but was flush with the deck surface and could easily be pushed up would also work. You may need to provide a ladder to climb up onto the deck Some egress wells have these built-in.

My building has some hinged grates that are held by locks that can be opened from the inside. I don't know that this would meet current International Residential Building Code, but is acceptable as preexisting by the Fire Department who has made us upgrade our smoke detector and alarm system several times in my residence here, but has not requested that the grates over the egress windows have their locks removed.