Can't record partial payment for pay-later event registration

This is a regression against CRM-13965, though I am not as familiar with this feature to know what exactly broke where/when: it just doesn't do what that issue says it does.

Anyway, the scenario is that someone has called in to register for an event (or, at that rate, has registered online with pay-later). Now, they have sent the check in for the first installment. There is no way to record that partial payment. Setting the contribution status to "partially paid" does not help.

I suspect the issue may be due in part to the code at CRM/Core/BAO/FinancialTrxn.php#L503 and following. If there's a contribution but no completed transactions, the function doesn't return any amount owed.