While selectDependencies allows to obtain triggers the given trigger depends on, the reverse operation is not supported by the API.
I'm using a couple of purely time-dependent triggers to mask particular backup windows, following the approach I described in http://blog.zabbix.com/a-workaround-for-trigger-based-maintenance/1527/. It's difficult to find all triggers that depend on it.
I wrote a quick implementation, that should be fairly efficient. It introduces a new option called selectDependent, which behaves analogue to selectDependencies. It is capable of returning triggers that directly depend on the given trigger.
My actual goal is to create ad-hoc dependency graphs. Creating greater trees requires as many API invocations as there are tiers of dependency.