I would oppose to any change that exlicitly undecorates and redecorates the window because (I'm summing up below what I already mentioned in a more haphazardly way above):
1. Both under Gnome and under Cinnamon there are scenarios in which changing the client decorated state wreaks havoc with WM operation. In Gnome when you remove decorations the client lose focus. In Cinnamon undecorating and redecorating a maximized window makes it completely inaccessible, as if the WM couldn't show it anymore. I managed to make the maximize-undecorate / unmaximize-decorate combo work under Cinnamon, but that's all. Expect problems under other DEs and WMs.
2. Using Jonathan hook shares the problems described in point 1, since it explicitly deals with decorations instead of just hinting the WM, but also fails to redecorate the window in many circumstances, since the hook seems to be called before the maximized property was removed from the frame properties alist. Even if there were a more convenient event to hook from (I was unable to find one in frame.el) I would disregard the solution because of 1.
So that let's us able to support WMs that offers _GTK_HIDE_TITLEBAR_WHEN_MAXIMIZED or a similar mechanism, or that explicitly advertise they allow decoration change events triggered by the client.
I've requested support for _GTK_HIDE_TITLEBAR_WHEN_MAXIMIZED in the Cinnamon tracker [1], maybe you can argue a bit more for the feature there.
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