@The Mole The URL you shared needs access permission. It would be better if you state the exact model number of the motherboard and post the request in the proper format as described in the first post.
Phoenix Tool 2.73 seems to add slic ok. https://forums.mydigitallife.net/th...slic-in-phoenix-insyde-dell-efi-bioses.13194/
@The Mole It's a typical AMI Aptio UEFI image. As @Flipp3r suggested, the Phoenix(/Dell/EFI) SLIC Mod tool should be enough to insert your desired SLIC in it using the default Module method. Try posting a RWEverything report if the Module-based approach failed, then we might need to do some manual hex-editing.