I think the only other way is to edit the autoexec.bat in the system root folder. This is a real throwback to the days of DOS ... but the file has remained. I believe the contents of this file are ignored by windows except for any SET commands.
To add a variable you would do something like:
SET MYVAR=C:\SOMEWHERE\SOMETHING
I suppose the challenge now would be to have something that changes the variable in the autoexec.bat before launching Windows. I'm not sure if that's possible.
Perhaps this link will be useful:
http://www.tomshardware.co.uk/forum/...5059_35_0.html
The last post in that link seems an interesting option. I think you could use it to specify a registry entry to run a .bat in the start of the Windows load sequence. Perhaps that would enable you to get the appropriate env vars pre-login:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager\BootExecute
It's been a while since I wrote .bat files but I seem to recall you can set menu choices which could be used for setting different options.
Sorry I can't provide an out-of-the-box solution to this and I wish you good luck.
Glenn