Well, I've started to add a basic pen tool (this is more exciting than setting the text of a placard :p). It flickers horribly, but it's a start.
(later) I've removed most of the flickering by restricting the drawing only to the areas that actually need updating. However this isn't working out that well, the update restriction code is a few pixels off. I'm going to leave this to tomorrow, when I have more time to dedicate to the problem, and a few hours to reflect about it on the bus to/from Brain Bowl.