Built-in support for drawing geodesic lines would be great (e.g. using http://fragger.github.io/Leaflet.Geodesic/). If adding that custom plugin for drawing would be easy, that would be OK too.
I’ll definitely need to make a reference for an add-on to add-on :-) Until you, me or somebody else makes support for this, I’d suggest to draw the geodesic line on the server side, not “real time” while drawing, but I guess works fine for most cases.