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 finally accepted my offer, I obtained it for less than 12000 yens (shipping fee included).

And while I was waiting to receive it, I looked for H-LINK compatible wired remote controls. But after a little bit of digging, I found that the RAC adapter can only be connected to what's Hitachi calls "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)

Looking for those model numbers on Google, I discovered that even the simple PSC-5S central station retails for 70000 yens and is a built-to-order product. The cheapest touch-screen type of central station retails for 135000yens... Nevertheless I checked on Yahoo Auctions and Ebay for a possible second-hand central station but only found one PSC-A64S (seems to be the English version of PSC-A64S1) for roughtly 11000 yens excluding delivery costs.

With some more digging, I learnt with this PDF that products starting with "PSC-" are only compatible with themselves. Kinda makes a lot of sense but with this new information I looked for PSC products on Mercari (the Japanese equivalent of Leboncoin/Craiglist/Gumtree) and found two PSC-A64GT2 for sales for less than 4000 yens each. Could that be the older version of PSC-A64GT3!? A quick search and I confirmed that indeed it was! Right away I made a 3500 yens offer to the seller and finally here I am: a RAC adapter and a central remote.