Segwit commitment to post-quantum witness data?

The discussion revolves around an innovative proposal concerning the non-commitment to PQ signatures within a blockchain's block extension. The idea explores the potential benefits of allowing malleable signature data, specifically in the context of SNARK aggregation. This alteration could theoretically enhance the architectural and mining efficiencies by simplifying the SNARKification process of blocks. Such a change is anticipated to reduce the burden on miners and potentially accelerate the PQ block relay process.

The feasibility of this concept hinges on several assumptions: first, that no PQ adaptor signatures are required, implying there's no necessity for proof of signature publication; second, that PQ signatures maintain a constant size, exemplified by something like SLH-DSA; and third, that the PQ extension block solely comprises PQ signatures without incorporating any other witness data. The core of the proposal questions the impact of signature malleability under these conditions. If the primary concern is merely the authentication of messages — verifying that a specific public key authorized a message — then the argument is made that the mutability of signatures post-transaction initiation or even post-block mining might not present significant issues. This approach suggests a paradigm where transactions with valid signatures remain valid, regardless of alterations to their signatures while in the mempool or after a block has been mined, thereby possibly eliminating the need for updates between parties regarding transaction signature changes.