I asked the guys at work here about this, using a DLL in an SSIS package, and one said that he had done this with DLLs that we wrote here. He did not believe he had to strongly type it (not something we've ever done).
So it is possible, but no one here remembers having the issue you are seeing.
You may have more luck in a SQL Server forum dedicated to SSIS issues, where this sort of thing is more likely to come up.
I'm assuming you ticked the Copy Local flag when you added the reference?