I’m polling now:
String Foscam_Detect "Bewegungserkennung Aussenkamera [MAP(foscam.map):%s]" <camera> (Status,Test) { http="<[http://foscam:88/cgi-bin/CGIProxy.fcgi?cmd=getDevState&usr=USER&pwd=PASSWORD:4000:REGEX(.*?<motionDetectAlarm>(.*?)</motionDetectAlarm>.*)]" }
Not the most elegant solution, and specific to Foscam, but it works and it’s fast to react, almost as good as a PIR sensor.
Thanks to @d0t for the hint. For API, see here.