I would agree that this is indeed a sensible error message when I were about to add a COM reference to an inproc (DLL) server. However, this is an out of process server, an EXE. This guy (callee) does load in a separate process space from the caller, with a separate instance of the .NET runtime. Someone care to enlighten me why this restriction is in place?
By the way, tlbimp behaves the same way (just to make sure...):
TlbImp : error TI0000 : System.Runtime.InteropServices.COMException - Type library 'OutOfProcServer' was exported from a CLR assembly and cannot be re-imported as a CLR assembly.