It's possible to spend about $100 and push a Steinberger Synapse guitar toward Klein-esque ergonomics.
The necessary parts (arm rest and leg rest) were originally part of the zoybar TOR guitar project, which is based around 3D printing.
Overall, this change is very comfortable. I think it would be only a moderate amount of work to devise a set of wooden or 3D printed mounting bases which could be attached to the guitar body using existing screw holes on the back of the guitar. This would preserve the stock state of the instrument, but in my case, I wasn't concerned at all about drilling new holes in the guitar. I knew it was going to make an instrument I was largely ignoring into one that I would play a lot.
You can check out my Valkyrie Superguitar™ here, which also features Klein-esque ergonomics and an array of luxury appointments.