A picture is worth a thousands words...
I look forward to seeing your code (he said in understated tones). Are you implementing Thomas Hein's "Flattening of Cubic Bézier Offset Curves"?
No, I'm using a brute force approach, iterating the strokes, finding the coordinates of the point at requested distance on the perpendicular, and so generating a huge number of lineto() that Gimp seems to handle very well (my code for this is a derivative of my Slinky plugin, for which I doubt there is a nice mathematical solution).
For the simple case we have here, I thought there could be a more subtle solution and your pointer is good news. However the text of the article on various sites is protected by a 30$+ fee and since I'm not expecting any financial compensation on the script I won't bother.