You could easily wrap the middle uplevel in a catch block to handle exceptions, and this sort of "with" construct is pretty common in a lot of very popular languages today.

While it would be nice if this were baked into the language, it's pretty astonishing how extensible Tcl is without even having to touch the realm of C extensions.

I think this precisely one of Tcl's strengths. Few other languages apart from Lisp and Tcl lets you write your own control flow syntax from within the language (and at runtime).