I assume it works if you don't do the packing?
Does it work if the .netmodule is present in the .xll directory?
The .netmodule file is a bit strange, I don't think it will work with the packing (which just handles AssemblyResolve events).
If it does work when the .netmodule is present (with the Tibco .dll packed) I suggest you store in a resource in your own C# assembly, and unpack it to the ?xll directory on startup(in your AutoOpen()). That should be early enough for Tibco.
Else look at the disassembly of the Tibco library and see how the use that file. Maybe that will give you an idea where to put it.