A working version after Big Sur is available here.
A Lilu plugin for enabling Sidecar on Pro SMBIOS devices (iMacPro1,1 and MacPro7,1). The plugin may be compatible with Lilu 1.4.0 or later and macOS Catalina or later.
This plugin is only tested on my Catalina 10.15.6 hackintosh with SMBIOS MacPro7,1, UHD 630 enabled and a dedicated card rx580. Maybe it will also work on those without integrated graphics with other patches.
The key patch is two hardcode board-ids in the framework VideoConference and AppleGVA.
-sidecardbgpro
enables debug printing (available in DEBUG binaries).-sidecaroffpro
disables the plugin.-sidecarbetapro
enables the plugin on macOS versions newer than Catalina.