VDP2 is based around tiles. I haven't found a good tile-editor and mapmaker for the PC, but there was a new Java-based tile-editor linked off of Zophar's Domain last week that looked interesting. I haven't had the opportunity to try it myself yet, though. As for mapmaking and other tasks, there's a professional DOS-era tool called t.u.m.e. that was released as open source a year or so ago. There are lots of other tools available as well, you just have to have the patience to try them or alternatively make your own. I don't know if any mapmaking tool directly support the Saturn's hierarchical tiling support, but it shouldn't be too hard to adapt existing tools (a map is a map, the elements can be of any size on a conceptual level).
