Using an El Capitan's OSInstall.mpkg should work too without issues.
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I know that to create a Catalina Installer (for unsupported machines) both your method and dosdude1 work, but also the
@ASentientBot one allowed to customize the Installer boot menu, adding for example the dosdude1 Mojave Post Install.app, that putting inside a Catalina BaseSystem.dmg will work , except that "Video Legacy Patch" needs to be updated with
SkyLight.framework,
CoreDisplay.framework,
IOSurface.kext and
IOHIDFamily.kext to reflect
@ASentientBot new Catalina patches, but
all the other retro-patches are Mojave confirmed.
The pictures showed are booting from a full (manually patched) CatalinaInstall.app (with all the dmgs inside as stock createinstallmedia) and pre-patched with shadow volume to allow inject the required modifications.
For InstallESD.dmg just this /Packages/
OSInstall.mpkg (HighSierra apple signed)
For BaseSystem.dmg just the "Mojave Post Install.app" in /S/Applications/Utilities/
and Utilities.plist customized
And on my root CatalinaAPFSVolume /System/Library/CoreServices/
SystemVersion.plist (from
HighSierra) to allow an in-place upgrade over a previous Catalina.