The <zone> element (part of the <facsimile> family) currently defines a rectangle on a <surface> by means of four attributes (@ulx, @uly etc.). There is general agreement among those working with <facsimile> that we need a way to define non-rectangular shapes on surfaces. This proposal is the simplest and most generally-liked option arising out of the recent long discussion on TEI-L and the graphics list.
@svg:points is a "list of points", defined here:
It's not really clear to me by what mechanism an attribute from the svg namespace could be added to a core TEI class when the facsimile module is loaded, though.