it looks like, with minimal effort, Fluke could release an alternative back that holds 2x 18650 Li-Ion cells in a north-south orientation, with spring-loaded contacts onto the cells so these are removable and can be externally charged. the diameter is an AA cell is around 14mm, while an 18650 is 18mm, so they only need to find an extra 4mm; the tilt stand design may also require reworking to accommodate this.
this is what i am looking at doing with a number of battery operated items around the home. good external chargers, like the NiteCore D4, are readily available and modern 18650 Li-Ion cells can hold a significant portion of their charge for longer than the lifespan of an AA Alkaline cell. it also seems that the world is filling up with spare (unprotected) 18650 cells that have good lifespan, left over from discarded laptop and electric scooter batteries.
cheers,
rob :-)