If the client can choose any mechanism it wants to create a verifier, the system you propose is (a) not SRP, as you concede, and (b) really just a clunkier way of doing 2FA†.

I like 2FA. Most people like 2FA. If you had started out by saying "passwords suck, the answer isn't bcrypt, it's 2FA", I'd have agreed with you. Instead, you said "SRP", which simply doesn't solve the problem you're purporting to solve.

† To see why: imagine your SRP-derived client-specified verifier generation protocol was adopted: you'd have three mainstream implementations (NSS, Safari, and Internet Explorer) based on passwords all of which would have the same security as simply doing scrypt on the serverside; the additional "win" of your proposal would be the oddball clients that did something other than scrypt, where SRP would give them a challenge-response framework they could layer their system on. Awesome. That's what 2FA systems do. And they don't rely on passwords at all.

If you're going to reimagine all of HTTP authentication, why on earth would you drag passwords along?