After discussing this suggestion with a few other scripters, I feel I've over thought the implementation of it, and even more so over complicated the explanation.
The end goal is a variable that is only accessible from within the script file its created in, by any part of the script file at any time. Essentially a script-file-global variable.
Instead of adding a new construct/command to create such a variable, an additional switch to either the /set or /var command would suffice, with the additional prop of ".scriptglobal" added to the $var identifer:
;Creates the variable "%varname" so its globally scoped to the script file:
set -S %varname value
; returns $true
echo -a $var(%varname, 1).scriptglobal
As explained in the original post, its precedence is between local and global variables, so the order in which variables are looked up is: local, script global, global.