OverviewAs you approach Lassen from the vicinity of Lake Helen, you will see a large rock outcropping on the mountain's south face. To the right of the outcropping is the South Face. The rightmost ridgeline is the SE Ridge (summer trail). The SW Slope is the broad snowfield to the left of the outcropping, and is the route one would take if traversing directly from Eagle Peak.
This route is probably best when well covered in snow and rime ice (the underlying rock is likely unstable scree).
Getting ThereThe usual approach during the winter and early spring is to ski or snowshoe up the park road from the Southwest Entrance (~6900'). Near Emerald Lake and Lake Helen (8164') make your way to the saddle between Eagle Peak and Lassen, around 8800' elevation.
Route DescriptionClimb the snowfield to the summit plateau (~10350'). I didn't measure the slope angle, but it seemed to be the usual 30-35 degree volcanic angle of repose.
Essential GearCrampons, ice axe, and probably a helmet would be a good plan (in case of rock / ice fall).
I don't ski, but it's probably a decent ski descent if snow coverage is good.
Avalanche DisclaimerAvalanche conditions in the park can be extreme during and after storms. There are several slide paths on the park road en route to Lassen Peak, and the SW slope could potentially slide as well -- don't climb it unless the snowpack is stable. An alpine start is advisable.
External LinksLassen Volcanic National Park website - during the winter months the NPS posts the snow depth / conditions, and in the spring there is a road plowing report. Call for avalanche advisories.
Lassen NPS web cam (SW entrance visitor center)
NWS forecast for Lassen, at the SW entrance