Despite people asking about them on forums and Reddit for years, I have yet to see a rear bike light with replaceable 18650 cells.
This site claims that Cygolite makes a few models with pass-through charging. This is the only place I have ever seen pass-through charging mentioned for a rear light.
What kind of brightness and run time are you aiming for? 18650 cells and pass-through charging from external batteries sounds a bit extreme for usual road commuters. Even in rainy, foggy Seattle winters I never felt the need for that kind of power in a rear light.
It's not what you're looking for, but my favorite taillight is an older version of the Knog Blinder Road. Simple, sleek, waterproof, and plenty bright for a rear light. It has an integrated USB-A plug that flips out for charging. Every few weeks I take it off my seatpost and plug in into my laptop at work.