Is there any supported way of adding to the built in Session properties (i.e session.props, not session.custom)?
I have some configuration stuff that would most naturally fit as session properties (to allow for binding support). I guess I can read/write to whatever session property keys I want, but I’d really like to provide schema information.
It doesn't look like it, no. It seems like a reasonable enough thing to enable modules to do (possibly gated behind a namespace or something), but nobody has asked for it to my knowledge so no such API exists.
Has anything about this changed in 8.3?
Nope 
I also made a new internal ticket, because I couldn't find one from the original report, which surprised me.