Does the WTO have a concept of “default” or standard tariffs for when a state has not yet agreed schedules? Or does everything default to zero?
As far as I can tell: no, there's no such notion of default tariffs. So they don't default to zero... or to any figure. There's one 2016 comment by its director-general obliquely answering this:
Britain is a member of the WTO and will continue to be a member of the WTO. But it will be [because of Brexit] a member with no country-specific commitments. We have had no other situation like that.
Normally tariffs (and quotas) are negotiated (with the existing members) when a country joins the WTO. (They can be renegotiated thereafter.)
Also, in the mean time, Britain has submitted its modified schedules (in July 2018), and is currently awaiting a decision by the rest of the WTO. Which should also happen soonish; as I recall the other members have 90 days to respond, which have elapsed or nearly so.