When you select t-slot edges, there's a dropdown list that appears and lets you select the screw size and length. Changing the screw length will make the "T" part move down. I've included the most common standard and metric screw diameters and lengths, but if there is particular size missing, let me know and I will add it.
.dxf and .xps support are unlikely right now. I haven't found any javascript libraries that implement it, which means I would have to write it myself. However, if enough people demand it, I could be persuaded.
In the meantime, you can use Inkscape or FreeCAD to import svg and convert to dxf (both are free). Inkscape's built-in dxf converter can have issues with AutoCAD. However, there are some Inkscape .dxf export extensions that work well. See
http://tim.cexx.org/?p=590 and
http://www.bigbluesaw.com/saw/big-blue- ... scape.html