I would love to know about this as well. It's a shame the Hive cannot always provide solutions for the many good questions it has in the Galaxy Help Zone. Given it is a difficult editor. It would be nice to know about other forums to look for help.
Before I present my suggestion, I must admit it's not a good one, and I really hate it myself when people write half-hearted suggestions on my problems, so I'm gonna apologize in advance...
If I remember correctly, the Kerrigan ability called "Wild Mutation" does provide some sort of extra layer of hit points as a separate bar, and not just bonus base hit points.
Surely the Behavior of that ability can be added to a unit as a permanent buff. If it works the right way, it should be simple enough to change the Vitals into Shield instead of Life.
But again, regrettably I'm not sure if I remember it correctly.