What about a NeoAir Jembe:
Cut a hole in the top fabric and use a wagbag, like the other options above. The Jembe itself weighs 7 oz, although it needs an inflatable pad (a NeoAir, not sure about other pads).
One problem would the need to inflate a pad anytime she needed to go, but it might be the lightest non-MYOG seat. Definitely trade-offs between weight and function for any of these options.