I suddenly discovered that Multires cannot autostart in my Win7PESE x64 build. It's even more strange 'cause the program start normally even under Win10 x64 (not PE however).
Looking for an alternative, I found a small and portable utility HotKey Resolution Changer (
http://funk.eu/hrc/). Just add it to your project autorun and use the hotkey Ctrl+Alt+R to display the program window. It allows to assign several hotkeys to different resolutions.