linewidth scaling

Can anyone suggest an approach to linewidth scaling?

I have code that does the following
vpath = art_bez_path_to_vec(path, 0.25);   /*bezier --> vector*/ 
trVpath = art_vpath_affine_transform(vpath, ctm); /*transformed vector path*/
svp = art_svp_vpath_stroke(trVpath,lineJoin, lineCap,linewidth, 4, 0.5);

and then renders the svp etc.

My problem is that we only have one linewidth so that whatever the
scaling of the whole curve the line width remains 1 or whatever I
set it to.
I can easily apply one of the scalings implied by the ctm, 
but that will only be good if the scalings are near each other.


I see that there is code under the hood (of art_svp_vpath_stroke)
that scales linewidth according to the slope of the line segments
etc, but I have no way to tell the svp renderer that the 'pen' is
10 pixels wide and only 3 pixels high etc.

It seems I could do something by copying & modifying a lot of the code in
art_svp_vpath_stroke.c and passing in the extreme points of a transformed pen
instead of assuming pen symmetry.

Is there an easier way?
-- 
Robin Becker