I think a potential problem with full hillside armoring is that in summer it will BAKE, and suffer the freeze/thaw cycle the rest the year. It might lead to durability issues. I suppose an armored area could funnel the water into a lined channel, maybe decrease the armored area subjected to sun etc.
Completely outside the box - another spillway could be in a different location, perhaps in the Bidwell Canyon area to the left of the dam. A canal or canal/aqueduct could convey water south then west to join the Feather River. Or perhaps the water could stop off in a newly constructed storage area before returning to the river, perhaps with power generation at the outflow - a mirror of the Thermalito setup. The elevation near the head of the canyon is higher than the MS and lower than the ES. Possible benefits would be improved flood protection, increased storage capacity, and increased power generation. Depending on the design of the storage area there could be improved recreation attractions, fishing for example. Of course, dollars are always an issue. Costs over time might be offset by additional storage and power. Repairing the existing design offers no improved storage or power. Just an idea.
A canal vs spillway might mitigate the “weathered rock” issue.