First of all, I think extensions like this need to go to the BIP7x (payment protocol) family of BIPs. BIP21 (Bitcoin URIs) are already too big and the industry is migrating to BIP70.

The idea of specifying the amount in local currency is not new and has been discussed before. I think the conclusion was always as long as conversion rates cannot be determined in a decentralized & reliable way, how would you deal with payer and payee disagreeing about the rate? What about fraud, e.g. a MITM injecting their own desired rate? At the very least I'd say it's tricky.

The "one QR code per price category" usecase is based on trust anyway – why not just up the address and let the user enter the local amount?