Hello,
thank you for the patch.
A solution which would not require named page sizes might be better
as we don't necessary need to know the name of the "previous" (or
"pushed") paper size (mostly we use "anonymous" page sizes, set up once
in the document preamble).
Thus push/pop way would be a bit more flexible (like
\pushpagebackground / \poppagebackground);
or making paper size local (enclosing paper setup into group -
\start \setupppapersize ... \stop\page) would restore the previous paper
size.
A normal environment should be enough (push and pop are used to save and
restore states etc.) which inserts are the necessary page breaks.