I don't if is a good idea. In the xml file there are stored the all options, window positions and much more. So it will be a suprise where the IDE and the windows arise.
If I would use your script, I would change it a little bit.
@echo off
set _DRIVE=%~d0
set DRIVE=%_DRIVE:~0,1%
%DRIVE%:\PellesC\bin\poide.exe /xml pelles-%DRIVE%.xml
where %~d0 is the drive of the bat file. (if you want to see more such macros, look at the attached test.bat)
But the best to fix this problem is to write a portable_poide.exe which replaces macros with paths in the xml - start the poide - when poide ends, replace the strings with the actual path to macros.
No great thing, but I don't have enough time on a piece to start this project (and I use each stick usually with the same drive letter).
Perhaps I use this discussion to start it anyway.