I personally like the idea.
Perhaps a2UST could be created by staking aUST to a specific contract, thereby making it immovable, but with higher yield. I do think for a2UST there needs to be some form of capital controls (lockup periods, etc.), otherwise this can be easily circumvented by a different protocol minting a wrapped, transferable version of a2UST.
This could fit the demand for the 2 user types of Anchor, aUST positioned towards DeFi applications with composability in mind and a2UST facing towards regular depositors such as third-party applications (which they don’t need composability)
aUST yield would still need a degree of yield flexibility, for Anchor to fluidly react to long-term market changes. This can go hand-to-hand with the dynamic Anchor rate.