Sorry, somehow I got it in my head that you were wanting to rewire something in your bathroom ceiling, like a fan or ceiling light fixture.

If your new wiring doesn't have anything to do with the bathroom, then I think it would be OK to simply have an appropriately labeled plywood cover in your attic floor to access that junction box. Just paint the plywood or lumber white and mark it with a felt pen.

If the new wiring is going to the bathroom light or fan, then I expect any electrical inspector that ever sees that wiring would want to see access to that junction box from the bathroom.