あっはっは。簡単だった。

dvipdfmx.def にある color関係のコードをそのまま xetex.def のもので
置き換えただけで、platex + dvipdfmx でも >>265 の例は綺麗に
描画できたよ。xetex.def のバージョンは次のようになってる。
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\ProvidesFile{xetex.def}
[2013/04/29 v0.96 LaTeX color/graphics driver for XeTeX (RRM/JK)]
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%