> "has anyone tried either an L-500L or L-500XL in the bridge position of a Les Paul?"

Lots of people.

> "How versatile was it?"

As versatile as in any other axe.

> "Also, 250k or 500k pots?"

As per the sticky thread: All the L-500's, except for the XL, are optimised for 250K pots. The XL requires at least a 500K pot. The higher the value, the brighter and more gain.

With a Q-Filter the XL offers more tonal variation than the L, but otherwise the L with 250K pot sounds similar to the XL with 500K pot.

You should select based on the best balance for the target axe and the other pups.

Aide toi et le ciel taidera (Aid thyself and heaven will aid thee)