You can make your non-GACed references show up in the Visual Studio Add Reference dialog by putting the assembly paths in one of the following registry entries:
[HKLM|HKLU\SOFTWARE\Microsoft\.NETFramework\<version>\AssemblyFoldersEx\
[HKLM|HKLU\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders\
The second version does not work in VS.2010 and seems to be deprecated. Is there any way to Get Add-Type to respect this list?
As far as I can tell this cannot be done. As such I opened a feature request on connect to allow for this in some cases.