Well, ask and you shall receive. I got in touch with the people involved in porting DRI to OpenSolaris, and it turns out they had recently released a snapshot of their efforts.
I got that installed this afternoon, but I'm still having some problems. Direct rendering is enabled, but I'm having trouble with AGP support (which may just be a configuration issue, I haven't played with it much yet). Performance still isn't quite where I'd like it, but it's nice to see progress.
I'll be following their progress with much interest.