I don't think it would make much sense, since it is the same as Fischer random just with restrictions regarding the starting position for the king. It's not as big of a change and is probably not worth the extra button and programming. Thibby likes to keep the interface clean and simple.
It's the one with simplified castling rules, right? I was interested in chess480 as well when I first got to read about Fischer Random because the castling in chess960 does seem a bit confusing at first (did to me, at least), but frankly, you get used to the 960 castling pretty quickly. What made it easy for me was to shift my thinking for a bit: in regular chess I thought of the castling in terms of how many squares the rook and king move -- but in chess960 the one thing to remember is instead the absolute position of the king when it's castled: it always goes to either c or g, and the rook of course goes past, to the next square.
ah, that's a good trick. In chess 960 I used to imagine what square would the king have been on, if it was normal chess - the middle square with opposite color. Then I imagined doing a castle from here by moving two squares, to finally get the destination. Indeed, it always end up being c or g. I will to it your way from now.
About chess480 Morozov has a point. I'll think a bit more about it, tho.