Hello Tony,
my personal two cents here: although I can see why one could need such a feature, I would not modify a standard tcl command for this. Chances are that this could get overridden or conflict in future tcl versions.
If this ever should make it to the openacs codebase, I would say it should go into a separate proc e.g. util::dict_get.
What is wrong with a oneliner like this? No procs involved and should do the trick
set value [expr {[dict exists dict noexist] ? [dict get dict noexist] : ""}]