I'm curious as to what the available lift capacity of a Mavic Pro might be. Suppose I wanted to fabricate a rig to carry and release a water ballon over a target - how much extra weight can it handle? Has anybody tested it?

It's not designed to carry anything - there has already been a very recent post asking the same questions...

You should not carry anything as it may cause the Mavic to crash.

Highlight below
Max load: 900 grams (2 pounds) (Not recommended, will damage battery)
Safe Max load: Below 600 grams (1.32 pounds) (WARNING: DO NOT fly aggressively)

Youtube is your friend....

About two pounds.