It turned out that my last dma fixes had a nice side-effect: Solaris 8 and 9 don't complain about spurious timer interrupts anymore. I'm really surprised they don't because I'm absolutely sure there is at least one bug in the system timer. Funny enough fixing the timer bug without dma, covered 98% of spurious interrupt complains, whereas fixing dma removes 100% of them. After all it's possible that both documents describing the timer in two mutual exclusive ways describe it correctly. There are just multiple variants of the timer chip.
With the Solaris 7 hack, Solaris 8 & 9 can boot in a single user mode, regardless the timer fix. Will update the status in the how-to shortly.