I always enjoy sharing stories of the humorous foibles of life. Politeness prevents me from sharing any about others, but luckily that still leaves me with plenty of material.
As for Einstein and the balloon, I dunno. I'd guess he would be amused but not be fooled by my cheap parlor trick. After he pinned inertial and gravitational mass together, it becomes difficult for me to think of ways around that. The balloon still has real atoms with real mass, so gravity must respond to the mass's energy. The accidental relativity of mass in Newton's equations doesn't carry over, as far as I can see.
By the way, after reading through a few other replies and poking at a couple of Wikipedia articles, I gather the imaginary part of the mass of various fields should be thought as more of a decay rate of unstable particles. Thus Gravity is excused from having to be extended to respond to the imaginary portion directly.
Keeping in mind my formal studies ended after the Bachelor level, is that a fair statement? Does imaginary mass pop up in any other guises other than a decay?