No, we don't have such an attachment, everytime I have brought this up I have been told if your tank is stratified and has a surface film, the flow is not sufficient and increasing flow and surface movement is the proper remedy.
One other option used for a store we supply with open top reef aquariums with surface tension scum due to inadequate surface agitation is to use the 3166 or 3167 filters while not a skimmer attachment I have found nothing better to skim the surface.
I thought about using one of those, in fact when I first saw them, i thought they were a surface skimmer for the 90xx family. In fact, they look like they might be easy to modify into a form of prefilter for the protien skimmer.
While I'm thinking sbout it, what's your opionion on using a reefpack 200 on a 50 gallon tank? I know it's the top of its size limit, but I'm going to a mushroom,zoanthid tank with a very light fish load.