This is not an answer but it does appear that a general filled polygon is an omission.
The line mesh and line strip mesh, although they don't meet your needs are behaving as one might expect. Each line segment is treated individually, not as a whole so the chamfering of finite width mesh segments isn't trivial. ... although could be done. One might also like the option for these lines to be finite thickness cylinders.
At the time I posted this, I spent some time on your website: lots of info to digest... In any case, having you hanging around here is certainly a good news for us taking a stab at dome projections. Thanks!
I personally don't think this needs to be too "fancy". A rectangular bounded region into which one can write a decent amount of text documenting what is going on.
Don't think it needs to be connected to nodes, one just needs to ensure it is selected when associated nodes are moved.