Agree Callum. At least as an option. Not everyone would like it though. But as you said, at the P and J models though. Good to hear that some of my questions to Sheldon are "fair enough". Otherwise, I usually get scolded for not being attentive enough (i e something already exists).

One other reason, is that at least more than a few posts/people, has expressed concern when playing near the bridge. The tone is good on the lowest strings, but as you go up to the last string, say, G-string that angle will produce a tone that is too close to the bridge and it will turn out a bit weaker. Then one has to angle the palm accordingly. I don't know if a long "rail" type thumbrest would mitigate this though.