GRC embedded Python vector support

[Marcus D. Leech]

So, I'm trying to implement and embedded Python block that handles 
vectors where the vector length is defined at runtime--like most
  blocks that take vector-length inputs.

But GRC only "recognizes" the I/O signature as being vector if the 
dimension is of a fixed size, and invariate with respect to runtime.  Is 
there
  a "trick" I'm missing?