Looks like the AVIC is not supported even in 16h family processors (the "Preliminary BIOS and Kernel Developer’s Guide (BKDG) for AMD Family 16h Models 00h-0Fh (Kabini) Processors" says that CPUID Fn8000_000A_EDX[AVIC] value is 0). This is likely the reason it is not implemented in the hypervisors either.
It also appeared that the second question was due to me using a different APM and IOMMU specification revisions. When I got the compatible ones (APM Rev. 3.24 and IOMMU spec Rev. 2.6), it appeared that the data structures for IOMMU changed significantly. Guest Virtual APIC Table Root Pointer moved from the DTE to the IRTE, and is now strictly defined as a pointer to the virtual APIC backing page.
A useful mechanism indeed, and it's a pity it's not supported in the hardware yet.