I have ScriptTrigger installed under the XTrigger umbrella plugin for a few triggers.
For ScriptTrigger triggered plugin if I access this URL:
http://<jenkins>/job/jobname/jobnum/api/xml?xpath=//cause&wrapper=causes
I get this:
<causes>
<cause _class="org.jenkinsci.lib.xtrigger.XTriggerCause">
<shortDescription>
[ScriptTrigger] and stuff printed by ScriptTrigger script.
</shortDescription>
</cause>
</causes>

I will have a look at adding support.