Thanks to all. UNC threads should be no problem as they seem to have the same geometry as Metric threads: http://en.wikipedia.org/wiki/Unified_Thread_Standard. Other threads e.g Acme will require a different cutter shape.

I'm satisfied when the final version of this script can make make metric normal and fine threads plus UNC and UNF.
I imagine
- a dropdown list where the different threads like "M6 x 1" or "1/4 UNC" can be chosen
- a input field for the thread lenght (which has to be slightly corrected by the script to allow full turns)
- a input field for the length of the not threaded part
- if that is possible by scripting the MoI units will be set to mm

I agree OSTexo making a cutting object that actually cuts as wanted is tricky. If the sweep overlaps or touches over a wider area you're out of luck.
I do it like shown below. The height of the cutting object is equal to the thread pitch and the sweeped profiles only touch at one edge.
The final trick to make this work was to move the profiles out by a tiny amount so the the boolean diff operation sees some difference.
I use 0.1 mm at the moment but I just checked that 0.01 mm seems to work too.