OK, finally I found it (also, got the sources) - the prog creates the registry entry (probably happens when calling CreateEventSource()), it was just not visible until refreshing regedit :-|
And, it DOES register a formatting lib, only that it is something I cannot rely on: C:\Windows\Microsoft.NET\Framework64\v2.0.50727\EventLogMessages.dll
Or, can I?
Ah, here we go for an explanation:
Difference between EventLog.WriteEntry and EventLog.WriteEvent methods
So, I cannot, as I'm not using .Net ... :-|
Now, if I NOW send another event from MY prog (with the same, newly created source, as the foreign prog), I see it in the event viewer normally.
That means, WriteEntry() actually does not write a different/special type of evt log record containing the text directly (contrary to the standard method of writing just a msg catalog ID + params) but rather there must be some trick in the formatting lib to make the EventViewer apply some kind of "default" formatting.
Any ideas how I could accomplish this? Except just copying the EventLogMessages.dll above? :-)
OK, finally found an answer about this one in http://msdn.microsoft.com/en-us/magazine/cc163446.aspx
"This file is called EventLogMessages.dll, and it contains 65,536 event descriptions, each of which consists of the string "%1", a placeholder for whatever string you want to write"
I was hoping for something like "messageid=*" but that seems to be too simple :-|
But maybe somebody else is interested in whats happening here ...