Ah. Understood. I solve this problem by forking every Quicklisp repo I use, and telling ASDF to look in my local clones first. This also solves the problem of making it easy to submit a pull request when I fix a bug in a Quicklisp library.
Also -- and this is very important to me -- it means I'm in control of which version of every library I use. I've been burned too many times by Quicklisp "updating" a library that was previously working which doesn't any more. Now I have direct git control over my version coherency and I'm a happy camper.