Thanks to Magnus who pointed me to looking at scripted fields. Take a look at: https://www.elastic.co/blog/kibana-4-beta-3-now-more-filtery

or

https://www.elastic.co/guide/en/elasticsearch/reference/1.3/search-request-script-fields.html

Unfortunately you can not use these scripted fields in queries but only in visualisations.

So I resorted to a workaround and use logstashs drop filter to remove the events I don't want to show up in Kibana in the first-place. That is not perfect for obvious reasons but it does the job.