I haven't found any github issues about this question either amongst the results i got, when i search for fail-fast. Now we are keep switching the :fail-fast? flag in our tests.edn, depending on the desired behaviour, which is a bit annoying. We did mark our integration tests with a ^:behaviour meta tag and we did define a kaocha test suite, like this:

{:id         :behaviour
 :focus-meta #profile {:default [:behaviour :x]
                       :ci      [:behaviour]}}

should be doable with a wrap-run hook which binds testable/*fail-fast?* to true/false based on the current test.