Painting the inside sounds like a bad idea to me, bits of paint flaking off into your oil isn't what you want! The oil itself is protection enough against corrosion.
There are windage trays on ebay.de from time to time, you could make something similar yourself. These will guide the oil above toward the pick up, but won't eliminate oil surge on those long left handers. Simplest baffle would be a plate with perhaps three cut outs in the bottom, you want to allow the oil to move from side to side and mix, but with more of a trickle rather than a slosh! Another option would be a trap door in the baffle, only allowing oil to flow toward the pick up but not away. You could weld windage plates into the sump as well.
This is all theoretical mind, I've not done it myself and haven't actually had a good look at many. I really can't say what the best option for a G40 is, perhaps some of the trackday and motor sport boys will know what works.