The way I do this kind of thing is:
(1) For new files, I have start parts, start assemblies, start manufacturing assemblies, etc. that have identifying parameters set to my name. Stuff like DesignBy, or other things that apply to the design stage. The DrawnBy type stuff might not be done by me. Someone else might end up doing the drawings, so that is initially set to a "-" or dash.
(2) I have a mapkey that sets the DrawnBy stuff to my name. I use this if I'm the person doing a drawing. I open the pertinent part/assembly file, hit that mapkey, and the values are set.
I don't know of any way to use Environment variables to set Creo parameters. Undoubtably one could use a Toolkit to build a custom add-on that would let you access this stuff. It's a common enough thing to do in general programming. I've used in applications I've written.
No matter what you do, though, you want to have it be something that sets the value and is done. You don't want it to be something that is applied every time a model is regenerated. That will likely cause havoc as the value is set to a different value everytime someone else opens one of the models. They might not even know they are changing things, but the next time someone opens the drawing it will possibly be incorrect, etc.
