If the mass fraction of batteries on the plane is bounded by some not too large number (I'd guess at most ~30%?), then your range increase will be less than that. Given the increase in complexity, risk and additional hardware neutralizing those changes, I don't think battery ejection (via gliders or anything else) is viable.

Mid-air refueling (e.g. via drones swapping batteries) would be more plausible, but it still seems like a risky and complex operation -- think of turbulence, weather events, remote flight routes, etc. (but maybe it's possible to get it reliable enough).