The TEI vocabulary allows marking non rectangular-zones:
<facsimile>
<surface ulx="0" uly="0" lrx="200" lry="300">
<graphic url="Bovelles-49r.png"/>
<zone points="4.8,31.0 5.4,30.7 5.5,32.2 5.8,32.8 6.1,33.4 5.5,33.7 5.1,33.3 4.6,32.2"/>
</surface>
</facsimile>
We need another decorator that will be able to draw such polygons.