grub4dos uses the drives that are seen by the BIOS.
Just before the E2B menu loads, the WINHELPER.USB file should be found and the xml files wiped using dd.
This is done because if you ran a WinPE.ISO file (say) next from the E2B menu, WinPE would see the xml file on the helper drive and run loadiso!
When RunWin8.g4b runs, it again should detect the WINHELPER.USB file (same as when the menu was loaded) and clear them and then copy in the correct xml contents (and modify the product key if needed).
Once the Win8 install ISO boots, we boot to Setup\WinPE. Now WinPE uses it's protected-mode drivers to find and enumerate all drives. Later when wpeinit runs, it looks for Autounattend.xml on all removable drives and interprets it (in our case this causes it to run LOADISO.cmd which loads the ISO as a virtual DVD).
So, the helper drive should be detected and wiped just before the E2B menu is loaded, if you go to the grub4dos command prompt from the E2B Main menu, you should be able to see the contents of the helper drive and list the files using cat. If you then ESC back to the E2B menu and continue with Win8 Install, it should clear and modify the \Autounattend.xml file on the helper drive (as well as the E2B drive) just before it chainloads (runs) bootmgr from the ISO file.
Is this what you are seeing now with v1.27?