Michal, if you consider to use jBPM console (or its server part) you can give it a try with REST interface. Once deployed you can use following URL to display all available resources of the REST interface:
http://localhost:8080/gwt-console-server/rs/server/resources
If I recall correctly you should use /gwt-console-server/rs/process/tokens/{id}/transition to signal you process instance where id is the process instance id and use 'signal' query parameter to provide event data to the signal method. When using this approach it will use 'signal' as event type.
So I would try following url to signal:
http://localhost:8080/gwt-console-server/rs/process/tokens/{id}/transition?signal=data
HTH