if i make a 64 bit binary and try to use that to document, it tells me that i need to open the 4.0 version of docx, which i do, but then in that version when i try to import it, it still says i need to use the 4.0 version..  ?

At present you need to document 32bit or 32bit/64bit assemblies. You cannot document 64bit only assemblies.

is this something coming in a future release possibly? not a huge deal and can be worked around, but it'd be nice not to have to carry around the 32 bit build..

Yes, this is something we are working on resolving for a future update. Just to clarify for anyone else reading this, a normal .NET assembly which by default is 32bit or 64bit is fine - it's only assemblies marked as 64bit specific (unable to run as 32bit) that are a problem.