box accesability

[infinity]

314.29--- but now i feel like I'm on the spot

I guess i see the fixture "as part of the building" and the box not accessible since you must remove it (the light) to get at the 4 square and whip.
Perhaps I should rethink this?

I would rethink it.

Have you even installed a high hat in an inaccessible ceiling with no access from above which has a JB attached to it? You remove the fixture and the JB can be accessed. Same principal would apply to your installation. Something like this:

[plumb bob]

excellent point. my excuse is I expect to find that j-box at cans but was surprised when i saw this one above a 2x4

[infinity]

For us old timers the JB adjacent to a florescent fixture in an inaccessible ceiling is nothing new. Back in the days before THHN and other 90° conductors we used TW. Since it was only rated for 60° C we would put in a JB above the ceiling and then run a 3/8" FMC whip into the fixture with higher temperature fixture wire.