After a few months of patience, I managed to buy the only RAC adapter that was for sale on eBay. Thanks to a slightly favorable exchange rate and a seller who accepted my offer, I managed to get it for less than 12,000 yen (shipping included).

While I was waiting for it to arrive, I also looked into H-LINK compatible wired remote controls. However, after some digging, I discovered that the RAC adapter can only be connected to what Hitachi calls a "central station," as shown in this products connectivity table:

(the "ワンタッチコントローラー (PSC-A16RS1)" only allows to turn the air conditioner ON and OFF)

While searching for those model numbers on Google, I discovered that even the simple PSC-5S central station retails for 70,000 yen and is a built-to-order product. The cheapest touch-screen type central station goes for 135,000 yen... Nevertheless, I checked Yahoo Auctions and eBay for a second-hand central station and found one PSC-A64S (which seems to be the English version of PSC-A64S1) for roughly 11,000 yen, excluding delivery costs.

With some more digging, I learnt from this PDF that products starting with "PSC-" are only compatible with other products in the "PSC-" series. It makes a lot of sense now, but with this new information, I looked for PSC products on Mercari (the Japanese equivalent of Leboncoin, Craigslist, or Gumtree) and found two PSC-A64GT2 units for sale for less than 4,000 yen each. Could this be an older version of the PSC-A64GT3? A quick search confirmed that indeed it was! I immediately made a 3,500 yen offer to the seller, and now here I am with both a RAC adapter and a central remote.