Apparently, I can just add Anaconda option to the kernel line in the /EFI/BOOT/BOOTX64.cfg. That line already contains another Anaconda option inst.stage2=hd:LABEL=Qubes-R4.0.4-x86_64 to select the installation medium.
Adding inst.text or rescue or inst.rescue does not work for me (I get a blank screen) but at least now I know where to put those options.