Here is a work-around that I've mapped but yet to ride: http://ridewithgps.com/routes/34336
I warned Adventure Cycling about 89, and they weren't interested. The Sierra-Cascades HAD to go over Mt. Lassen. By using Google satellite imagery and maps, you can figure out variations to my route and check for viability of lakes/streams/etc. My variation involves some dirt--but nothing extreme. It will be wilder than 89 but quieter by a huge degree. I'm really looking forward to riding it. Monitor Pass and that area is GREAT, so go for that. I've ridden it several times fully loaded. Shoot for a wonderful forest service campground right near the town of Markleeville, although it will likely be packed on weekends--no hiker/biker sites. There are a couple of good restaurants and a market in town. I've ridden virtually all the route you're talking about from Bishop to Mt. Shasta. It's just that bit between Truckee and McCloud that's the problem. Do some work on Google, and you'll be amazed at the options. A water filter is recommended for the remote areas I'm talking about.
Go for it and report back!