I have a sort-of solution. It’s still a huge hack, but at least it’s a self-contained hack that only needs to be set on the parent blueprint, and allows me to set up the children as normal through the blueprints component screen.
Basically, the parent blueprint has an empty static mesh component, and a construction script that looks for the first child static mesh component, copies all of its properties up to the root mesh, and then deletes the child mesh.
One caveat is that not all of the properties on the static mesh are available for getting and setting.
The ideal solution would be if the engine supported “code-only” blueprints with no root component (similar to a “blueprint script component”) that could be inherited from by a normal blueprint.
Additional: There is a feature request from around a year ago - UE-2584 - to allow altering of inherited components, [as mentioned UE4 staff here][2]. I assume this feature was never implemented.