My MFT.LSP or "Multiple File Tool".
This is a lisp-program (its to big to just call it a function, imho) that will allow you to select files (in pretty much any way you might think off) & select a lisp file and/or enter a macro, script code or just 1 or more commands (command ...) to be executed on each file.
When starting the function you get the main dialog:

Here you can set the various options how to handle certain situations, progress bar on/off & logfile.
supply a .lsp file to load and/or supply the code you wish to run.
For the example i already added the requested sysvar change as code. Which should be enough in this case.
The code can be edited inside its own edit window:

Short of full fledged lisp functions you can enter almost anything. (including macro's !)
Which came to be with some help from @john.uhden

After (or before) that you can select your files:

You can add files in any way possible. Add (all) currently open, read a single dir, a tree or even from a file (Note: This would require import filters for most applications, like excel). You can save/load/edit the filelist. Change the order or just sort it alphanumeric. It allows for filename wildcards too. (only for adding. Not to filter the list itself)
During the process it will check each file real-time (read-only? open? openByOther? ...) and allow you to take action.
There is only one minor 'downside' to it all (if you can call it that): it requires DOSlib to run.
You can find some more information on the function and how it works >here<