This might have been the case in the past, but not any longer. ANA’s seat map will lock out under My Bookings, but they allow seat map access via their check in pathway once you have checked in with UA initially (don’t need to wait for UA segment to be flown). Other airlines like LH-group and CM still have fhe restriction as their websites automatically redirect you to UA if you try to check in with an unflown leading UA segment.

On a related note, as far as I’ve seen, UA’s OLCI system doesn’t give seat map access to any OA flights and if trying to accomplish online, would need to be done direct with operating carrier’s website.