I have finally written a very simple user script that redirects you from Yahoo! JAPAN Auctions page to Buyee proxy equivalent page:
yahuoku_REDIRECT-TO-BUYEE.user.js (install with Raw button, hidden in “…” menu on mobile)
location.assign(location.pathname.replace(/^\/jp/, "https://buyee.jp/item/yahoo/"));
You should of course only install this script if you are geo-blocked (as me, in Europe).