Thomas -
As it turns out, a couple of developers on the team are working to prototype a custom processor which might do what you're looking for, but the proof of concept is not working properly yet, so it's not something I can share.
So, let's see what we can do with what we have available out-of-the-box...
I changed my FieldEnricher to enrich events received from the feature service's feature class, rather than the non-spatial table.
[ATTACH=CONFIG]33128[/ATTACH]
The two filter elements are branching the event flow one way if the EventCount field exists in the received event (bottom branch) and the other way if the field is not included in the event's attributes. Notice that one filter specifies 'AND' while the other filter specifies 'NOT'.
[ATTACH=CONFIG]33130[/ATTACH]
I've attached a 10.2.1 product configuration I exported which defines all of the various elements I created for this exercise. I had to wrap it up in a ZIP to get the forum to allow me to attach it. You should be able to unzip and import it into your GeoEvent Processor to create the Inputs, Outputs, GeoEvent Service, and GeoEvent Definitions needed - along with a registered Data Store pointing to LOCALHOST where the feature service is assumed to have been published. The GeoEvent Definition trail is especially important to getting the event flow to work. I didn't use any Field Mapper processors to ensure that the event schemas matched what was needed to update the feature service. You will want to look at the GeoEvent Definitions being created by the Field Reducer, Field Enricher, and the Field Calculator named 'ResetField' which creates a field named EventCount if the received event does not already have that field.
I included some sample JSON data in the ZIP archive with the product configuration.
- RJ