I have written a little step by step tutorial how to build a portable version. If this works for you all, I will put this or a corrected version to the wiki.
I have read your make-portable.txt.
1.) I am not sure why you use their a ..\usr\.. directory. I think the common directory for all is ..\PellesC\.. and you don't use it in your batch.
2.) You write, that the drive letter has to be changed. I (\PortableApps\) and perhaps other will use different directories, so the description will be more common, when you write you have to adapt the first time drive and path.
I think, it is a great idea to write such thing into the so that it became a knowledge base for Pelles C!