> I’m similarly curious to hear if other people have done annotation with 
zoomable interfaces before. 

I have been trying OpenLayers' stock functions for this kind of thing for 
OurDigitalWorld, there is an example here [1]. Leaflet probably does this 
well too. The mapping tools do seem to have some slick drawing functions 
[2] though I have only used polygons.

art
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1. 
http://tiles.uwindsor.ca/ink/cecil/focus/swoda/pictures/assumption/09_1915/00001
2. http://openlayers.org/dev/examples/draw-feature.html