I've posted a new Leveller 2.6 patch available at http://www.daylongraphics.com/products/leveller/download/updates/lev26_7322.zipThis has modifications to both the BT driver in GDAL and in the BT driver handler in Leveller to support void elevations (assuming your BT files use the official void value of -32768).
When extracting, just place the two files into your existing Leveller folder. To be able to revert changes, it's recommended to copy or rename the original files before overwriting them.
A fix for the earlier problem of dragging and dropping image files onto the scene pane is also included.
Ray