Seems like this thread hasn't had much activity since before the pandemic, but I have a similar question. I'm flying into HND on AA 169 in J (and FWIW I'm also AA Plat/OW Sapphire) - since I'm landing before 5AM, I'd love a shower before heading into town. My understanding is that there still is no arrivals lounge at HND and the TIAT shower room is my best bet.

The site linked above says "TIAT Shower room is available at Arrival lobby on 2nd floor. Customers arriving at Haneda on JAL operated International flights with JAL flight number are eligible for the service on the day of arrival." so it looks like I'm SOL since I'm arriving on AA. Worst case scenario, I'll pony up the 1500 yen, just curious if anyone has had any luck gaining access on a non-JAL operated flight with non-JAL OW status... otherwise I'll wait a week and report back with my own findings!