Yep, it's possible to add another parameter for the fog density between z=0 and z=1.
Honestly, I'm not working on adv mode now, but eventually I will. There are some difficulties with it, so the way of rendering above levels will be the least of them.
The most important thing I want to know now is about the branch with non-square text tiles.
Obviously it provides better user experience, has cleaner, better structured and faster code (because map rendering is separated), and I personally want to focus on this branch.
However there are some difficulties. First, special versions of some plugins will be required (at least mouse query, building plan, then rendermax again). I'll make versions that will work both with and without twbt, and will provide a way for plugin authors to know if twbt is running in this mode and get required data from it. But still this may cause some complications for players and package makers.
Another problem is that apart from the maps in fortress mode and adv mode (square tiles) and text (non-square tiles) there are also minimap, maps on embark screen and so on. Should they be in text? It's not a problem to have one more separate font for these areas but it still will be rectangular and sized as all text.
So basically I'd like to know how important non-square text is and whether the community is ready for some inconveniences like incompatibility (likely temporary) with some plugins that this functionality may cause.