This feature is not in the WED developer preview (because I just coded it) but: WED 1.1 will feature "click-to-split" for edges. With WED 1.1 you can option-click the edge of a polygon or line feature (but not a must-be-straight entity like a runway) to insert and drag a split point.
This will hopefully be a lot easier than the current, convoluted, WED 1.0 technique of selecting the two surrounding vertices and picking "split", then repositioning the vertex.
Hackers: this features is not yet checked into the tree, so ... building from source won't help you. It'll be available some time in the next week.