(1) Oroville's design is stuck with the 1,730 ft long 901ft elevation crest for the original 350,000 cfs "Emergency Spillway" requirement for PMF (now bumped up to 369,000 cfs by the BOC). DWR has played PR "name games" switching auxiliary spillway with the original design stated "emergency spillway". Oroville's design is 100% accurate to "emergency spillway" per definition of 3.2.1.3 above.
(2) It is physically impossible to have an "auxiliary spillway" function (as stated in 3.2.1.2) unless a linear section of the 1,730 ft is rebuilt to a lower elevation, thus preventing the 901 ft elevation "emergency spillway weir" from self-activating.
However, IF cash were flowing in heaps, and there was a luxury of time, a redundant "less sophisticated non-radial gate" chute lined w/ sidewalls parallel structure could theoretically serve as an "augmenting to a service spillway discharge capacity" (i.e. augmenting the main spillway). This would kick in at a lower elevation than 901ft with perhaps a simple release mechanism (other engineering technicalities arise due to the fixed main spillway head gate structure & Head requirement for flow, but ignoring these complications just for discussion sake).
(3) However, the sharp elevation drop to the Feather River, due to the tremendous height of this dam, creates a "solution killer" in that erosion must not dam the Feather river. Severe damming of the Feather river causes a backflow to the toe of the dam. This can flood the power plant, thus *failing* the turbine infrastructure & losing the ability to drain the reservoir below 813.6ft.
So - (1) without full hillside armor will cause (3), (2) will prevent (1) from fulfilling the spec requirement of rated 369,000 cfs, but (2) will still cause (3) without full hillside armor or a "walled chute". (2) will require big cash & time on top of nearly an impossible window. (2) cannot handle a PMF flow, thus (1) has to kick in and you have (3) again (unless there is full hillside armoring). Also, (2) cannot fail in concert with (1) as it must handle a PMF flow.
No matter how you look at it, the only solution with time & wise spending is to fully armor the hillside (& buttress the concrete weir to secure its anchoring). Only this solution will protect the main dam in a full "safe" redundancy.
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.