I only tried it on a couple titles, and they worked. I do recall there being a limitation, accessing menus, no fast forward, something like that. I have a hardware hack for my Oppo, so that's what I use.
In the past, all Panasonic BR players had a key-sequence hack for region free playback. I think it might vary by player. I have a couple older, cheap Panasonic players and it worked on them. No idea about the UHD models.