All great posts, thank you!
Echo17's stroke based implementation best matches the slightly-modified idea mentioned above, it can easily recognize shapes being drawn from any combination of directions, so I'll probably use that. It is the first library linked in my original post, Drawing Engine.
The other engine, HyperGlyph, also still stands out - but hasn't been updated in over two years. Drawing Engine was updated two weeks ago, so it makes it a much easier choice.