The bad news is that the current version TraceEventParser does not support the scenario you want.
The good news is that the next version of TraceEventParser does support what you want (it is a subclass of DynamicTraceEventParser that will parse any event that was registered on the machine (whicn include all the OS ones). This
code already exists internally, so it is just a matter of our release schedule.
Exactly when the next version will come out it still to be determined. Certainly sometime before the V4.5 version of the runtime ships, but more likely well before that.
Along with that, we may publish a tool that generates a TraceEventParser from a ETW manifest (thus you can create strongly typed C# for Microsoft-Windows-HttpEvent, just like the Kernel and CLR events).
In the mean time you can often muddle along with 'by hand' parsing of the few critical events you want to parse (using the 'Opcode' ProviderGuid and TraceEvent.EventData() operations). This is painful, so only do this if you really are stuck.