I am also in the middle of a run through, doing what you suggested before (a portable installation of 2.4, followed by overwriting the files in that install with the ones downloaded for the 2.5-Beta stuff). I will keep an eye out here.
In the meantime, can you tell me what files are actually supposed to be different between a standard and a portable installation?
I've found all the .INIs and checked all contents, to make sure I pick up everything pointed to, and I've been going through the old standard install for ref's like registry entries .. but it'd sure be nice to have a simple list of those files that are specifically changed for a portable installation.