Here is a duke3d.exe and build.exe and mdsprite.c with the fix for anyone who wants to play with visible turrets.
Note that the floor aligned sprite bug is not fixed in included exe files so you will be seeing posters standing up on the ground etc on user maps and original maps without the HRP maphacks.
All changes to the source is marked with comments starting with "Parkar:".
Quite essential for everybody but there's a slight problem.
If you notice weapons HUDs (when using this patch) the Devastator and the Laser Trip Bomb, they show Duke's right hand have risen up a bit. But at least, effectively, the turrets are all visible as supposed.
parkar at
The HUD models being moved is just a side effect of fixing the ceiling models since both have the y-flipped flag set.