John,
what would happen technically when heating up a TSUBA to melting temperature? We are talking about 1.538°C.
As you cannot suspend the TSUBA on a wire or so in a furnace, you will have to place it horizontally on something flat that would be more heat-resistant. As all metals are very good heat conductors, the temperature on the upper side of the TSUBA will be the same as on the underside. So while you are trying to create a "like-melted" texture on the upper side, the underside will do the same and take on the texture of the material it is lying on.
Now the details: A clay-based ceramic board that could withstand temperatures of about + 1.500°C was not known in Japan and is not known elsewhere today. In high-temp ceramics like porcelain, silicon-carbide boards are now used which withstand temperatures up to 1.700°C (theoretically). In traditional porcelain firing in China and Japan (about 1.350°C), special fireclay-based boards were used which had a short life-cycle.
So I am afraid we have to discard the notion of YAKITE SHITATE being a "superficial melting" process in TSUBA making. As the term YAKU implies, fire plays a role in the process, but it is not aimed at melting.
Coming to GAMA HADA, we should have a look at a toad's skin first (see images). I am not speculating how it would have been made on a TSUBA; there are several ways to do it, and none is fast and easy.
Looking at your TSUBA above, my comment would be (not looking at the NAKAGO ANA shape which is off) that it was made by casting in coarse sand. You can buy TSUBA like these cheaply as souvenirs (see images of my souvenir TSUBA below).