Hello Andy,
did some tests:
1) normally placed surface (2S) -> then extrude edge
2) surface (selected as surface or object) switched to 1S (my normal work-mode)
-> then extrude edge -> added surface = transparent -> CORRECTION: in this case the normal was flipped
BUT (picture above, yellow circles) flip normals doesn't help -> "real" transparent (should be white(1) ??), but texturing possible