Yes, with the same memory modules and motherboard BIOS version Multibit ECC works with the 7800X3D as expected:
Also nice that error injection works, that makes it easier to practically test if ECC works correctly, on AM4 I never saw this feature working. On AM4 the most reliable way to check if ECC is functioning correctly is overclocking the ECC memory to its limit without increasing the memory voltage to force memory errors which then get corrected and this action is subsequently logged in the memory testing program.
Stuff that also just works after swapping the 8700G out for a 7800X3D:
- IOMMU Options
- PCIe Bifurcation Options
Still sucks that on AM5 ASUS seems to be the only reliable option left regarding ECC memory support, ASRock manufacturs many of its motherboards with the necessary additional traces for the memory modules but then messes up the BIOSes regularly 
On AM4 ASRock, Gigabyte as well as ASUS made motherboards with ECC generally working, only MSI was the negative exception.
As mentioned earlier the 8700G gets returned, feel a bit dirty… but on the motherboard I have which functions completely fine including prime95 with AVX-512 with the 7800X3D it is actually unusable in Windows with the 8700G, not even thinking about ECC, PCIe Bifurcation or IOMMU options.
Hopefullly by the time the PRO 8750G or whatever it is going to be called is going to be released, BIOS and driver updates will have improved the situation.
Tested AM4 APUs pretty early after their initial releases (3200G, 4750G and 5750G) and the situation there was never this bad.