Unless I'm mistaken, any owner can work on or modify a homebuilt that they own. You have to be the builder, in compliance with the 51%, rule to be able to do the annual inspections yourself. The trade off is that if you sell a homebuilt that you put together, you are the manufacturer. I'm sure that there is some liability.
It would be nice if there was a course and a test you could take to be able to work on your own certified aircraft without having to get an A&P to sign it off. The trade off here would be that it might be smart to undo any modifications that you might make before selling the plane.
