CS5 allows you to save FLA project as a bunch of XML files that encode vector graphics in some format, like:
<Edge cubics="!8809 4331(;8821,4327 8849,4316 8865,4311q8809 4331 8865 4311);" />
Is there a format specification somewhere explaining what those entries mean?
Not yet. Adobe has said they intend to release a spec, but it doesn't seem to have happened yet.
Unfortunately I also don't think it's currently known what those numbers mean (unless you figure it out!). But I can make an educated guess that the numbers themselves are probably in twips (which in Flash-speak are twentieths of a pixel, not of a point as in typography), so if you divide them all by twenty you'll probably have pixel offsets measured from the upper-left corner of... something. ;) Which may be enough for you to decipher what that line means.
But right now, the best we can do is interpret a given piece of XFL; there's no way to completely know in what way it is or isn't legal to change it.