What about just creating the map entity after the boss is dead instead of switching from passable to solid?

My client wants to be able to place the object in the level editor before it appears. But anyway, I have found a solution:

I had forgotten this, but in my player code I set the player's push value to -1 to stop his own bullets from damaging him. So, when I attempted to use a work around such that the wall has a -1 push value and the player uses IGNORE_PUSH, it didn't work.

Of course, now that I've realized these things, I instead set the wall entity's push value to -2 or lower. The player can now pass through it without any problems. It seems to work just as good as the PASSABLE flag, although I'm still somewhat bewildered over why the PASSABLE flag seems to have no effect on map entities.

Good to hear but the PASSABLE flag does work with map entities, I use it in my own project!

Perhaps the PASSABLE flag is affected by the push value of any colliding objects..? This would explain why it didn't work for me, as my player entity had a lower push value than the map entity.