I had a similar situation a few years back using a DOS boot from a USB stick - sometimes the boot would be to A: and sometimes to C:
The solution was to use a batch file to test for a marker file in each possible drive letter and when found, you would then progress to the appropriate set of switches for that particular drive assignment.
Have recently done similar for a WinPE environment using Imagex rather than Ghost, but the principal of searching sequential drive letters for a marker file still worked perfectly, this time in vbscript.

EdT's solution is the only way I've found to do this as well.  (My script checks the drive label, but the gist is the same.)  For some reason, I cannot get the @CD switch to work.

That being said, another thing I've learned:  You'll notice the default commands created by the GBW include a call with a /setosdrives switch.  Be sure to find the drive letter AFTER this command is executed.  In fact, I moved this to the top of my script.

The problem I kept running into was I would find the drive letter, create a parameter file, then execute the two lines created by GBW (using the parameter file on the final ghost32 call).  Everything would work great up until the point where Ghost tries to access the image file, then it would fail.  I finally tracked it down to the fact that /setosdrives was rearranging all my drive letters!