Wow, vram_pushbuf=1 worked like a charm! Please send patch upstream and/or change docs now! Explored every nook and cranny of Bios for the nth time, for n>10, and found no mention of iommu or agp or any iommu-sounding-like thing. Will post dmesg and lspci for the successful boot, shortly. I'll also try to pull syslog for the previously-failing boot.