I too hope an electric UL will someday be reality. But I understand the FAA's decision to not consider batteries the same as five gallons of fuel.

One reason for the 254lb restriction is to limit the amount of damage a UL can inflict when it hits something. The amount of energy dissipated in a sudden stop goes up dramatically as mass increases. Higher gross weight also makes it difficult or impossible to meet the UL stall speed requirement.

And......lithium batteries burn with incredible intensity when they are ruptured. I suspect the FAA is hoping lithiums are superseded by something with less fire risk.