I'd say the community hasn't answered because it's a border case not many got an opinion about. And Particle is just not as much around in this forum as some of the Elites are (too much 
)
I have thought about your proposal but haven't really come up with a "reliable" answer.
My gut feeling is just a bit funny about this, but can't prove any (also because I'd need longer testing to convince myself).
Without having looked into the Electron schematics and any number crunching, I just got the feeling that the AAs would get drained first till their voltage equals the one of the LiPo + 0.2V (schottky forward drop) and then both would get drained in parallel (with the risk of back feeding into the AAs if you don't add a protective diode).
So not what you want to achieve.
But @BDub is the master of the electrons on the Electron - I'm more a bits guy in this respect 