If you copy the normal inverted ramp corner and change:
COVERAGE: //TBNESW
1 : 4
1 : 1
0 : 6
0 : 6
0 : 6
0 : 6
to
COVERAGE: //TBNESW
1 : 1
1 : 4
0 : 6
0 : 6
0 : 6
0 : 6
then it looks like an inverted ramp brick. But as you'd expect from a very simple change, there's problems - no collision whatsoever (can't hammer), for some reason no rotation when placing, and it says it's "Floating" unless you place it to the side of a brick.
If someone can figure out how to edit the rest of the file...