I don't know if this will work in your situation, but it's what I use.
I got used to using Fcc with mh-e, which copies to MH folders.  My MH
folders are now my nnml groups, so I wanted an Fcc that would both
copy to the folder in MH style, but also update the overview file so I
wouldn't have to run nnml-generate-nov-databases.  I came up with the
following after poring over the nnml source.  I'm not sure how safe it
is to count on nnml-request-accept-article's calling convention but
it's the best I could find.

Note that I decided to grab the Fcc behavior from mh-e so
I'd get completion on group names.  That requires a bit of mucking
with the resulting string (have to convert from x/y/z to x.y.z) but to
me it's worth it.  If you don't use that then your
message-fcc-handler-function can be made much simpler.