At this point, the only way out of this conundrum that I can see is to either hand parse the pattern (erk) or use a Safe compartment (re-erk).

Well, for starters, the user doesn't really benefit from \U or \L - it only requires the user to type more characters. For \Q, one could consider adding a flag to the program - if the flag is given, the string should be searched for as is (that is, as if \Q was prepended at the start of the search string), or else, as a regex (kind of like the -F option for (GNU) grep). Else, you can always use something like:

    s{\G([^\\]*(?:\\[^Q][^\\]*)*)
        \\Q
        ([^\\]*(?:\\[^E][^\\]*)*) 
        (?:\\E|$)}
     {$1\Q$2}gx;
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