There is an excellent ski touring book of Lassen Volcanic National Park called, oddly enough, “Ski Tours in Lassen Volcanic National Park” by Marcus Libkind (Mountain Sports in Chico at 176th E. 3rd Street, Downtown Chico usually carries it.)
The Southwest entrance to Lassen Park is commonly used by Chico locals. The road is normally plowed just past the entrance station. You can ski up the road for quite a ways, or head straight up the hill just past the Sulphur Works Hot Springs. You’ll smell it before you see it, and where the road takes a turn to the right, head straight up the hill. You’ll come out at the Diamond Peak saddle and once again catch the road right on the other side of the saddle. Some good tele skiing is available up at the top of this saddle. By the way, be prepared to remove your skis at Sulphur Works — the underground hot springs keeps that section of road snow free most of the winter.
Later on in the Spring, the road is commonly plowed up to the Bumpass Hell Parking Lot, which is only about a mile below the Summit Pass, which is the trailhead parking lot for Lassen Peak. Skiing abounds up near the Summit Pass.
From the snow covered trailhead parking lot you could climb the ridge towards Lassen and find lots of skiing just over the ridge, or head across and beyond the parking lot for a bunch of small bowls and slopes to play on.
There is an awesome webcam located at the Southwest entrance visitor’s center. Here’s the link to the webcam page.