A new Logstash filter called logstash-filter-percolate is now available to percolate events on the fly.
The original motivation came from this issue and the need to leverage the percolator while processing events in Logstash.
The filter is available here: https://github.com/consulthys/logstash-filter-percolate
It is worth noting, though, that this filter is only useful if you're using ES 2.x and earlier. The reason for this is that in ES 5.0 alpha2, the Percolate API has been deprecated in favor of a new percolate query (see https://github.com/elastic/elasticsearch/pull/17560) which will run like any other normal DSL query, and thus, allow the use of the existing elasticsearch Logstash filter in order to percolate documents on the fly.