Any chance you could provide some info on how you did this? I can build VirtualBox from source, but the only instructions I've previously seen for keeping the BIOS file up to date involved hexediting the bios binary. Maintaining a patch against the source code seems like it would be easier.
CustomTable is already in 5.2. Actually, SLICTable is not mentioned anymore in the manual and only kept for backwards-compatibility. It can safely be replaced now.
Are there additional steps required to get a custom SLIC to work where the host is Mac OSX (running on Mac hardware)? I've tried on both 5.2.22 and 6.0.0 and both seem to be ignoring the ExtraDataItem entries in the vbox file. Even deliberately fudging the SLIC filename doesn't throw an error, nothing appears in the logs and RWEverything shows the standard VBOX stuff (and no SLIC table) despite the VboxInternal/Devices/pcbios/0/Config/Dmi commands.
Being a major release, almost certainly. Are you using the correct entries? Code: <ExtraDataItem name="VBoxInternal/Devices/acpi/0/Config/CustomTable" value="path/to/your/SLICTABLE.bin"/> DMI is only for SLP 1.0 (Windows XP).