This is pretty much how I do it as well.
1) Download the
macOSUpd10.14.1.RecoveryHDUpdate.pkg (thanks MisterAndrew PM me the link. Just learnt how to search the SUCatalog. Will do it myself next time. Thanks tsialex posted the SUCatalog link, and of course, share all his findings)
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2) Right click the file, choose "The Unarchiver.app to unzip it.
View attachment 793383
3) Inside the "macOSUpd10.14.1.RecoveryHDUpdate" folder, double click RecoveryHDMeta.dmg to mount it
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4) Then double click BaseSystem.dmg to mount it.
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5) Double click this "install macOS Mojave Beta.app" to trigger the firmware update.
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6) Then just the usual stuff. Follow the on screen instruction, click shut down, wait 10s, press and hold power button until the long beep, release power button and wait for auto reboot. Anyway, as expected, this can be down with just the RX580, no Mac EFI GPU is required.
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Because he want to utilise all 4 slots