Perhaps buoyancy could be a decent substitute, at least for the solid waste part. I imagine being waist deep and flushing the entire bathroom after each training session. Maybe some kind of spatula/squeegee might assist with separation, coupled with a robotic spatula cleaner and sanitizer. There would be a monitor and cameras so you could calibrate your aim. What an odd workday that would be.
The goal here is neutral buoyancy when in gravity so that it behaves as though there were no gravity. Put a bag of water in water and it floats like the rest of the water, gravity or no.
So you’re strapping yourself into a material with the same density as poop and then pooping into it? How is that cleaner than pooping in a bag or over a vacuum?
Neutral buoyancy is achieved with very specific densities. You can either make the astronaut buoyant, or you can make the poop, but not both at the same time.
Do you need both? I assumed the astronaut has a handle or strap or something to fix their reference frame to the toilet. They can be only partially submerged.
https://en.wikipedia.org/wiki/Reduced-gravity_aircraft