I'd be very sceptical about kidneys not working whilst inverted. If you had fairly severe pre-existing disease with marginal blood supply to your kidneys it might exacerbate it. As would being stuck in a hot canopy without a drink.
Kidneys presumably work reasonably in zero G or else astronauts would all die. David Blaine did a stunt a few years back where he spent a few days upside down. He had the occasional break but as kidneys don't like being starved of blood for even a few minutes, if it had a major effect he would have suffered long-term damage anyhow.