The common thought in the NFA community is that once you hang an AOW masterkey under a rifle, it becomes an SBS due to the rifle stock. I believe that the masterkeys are sold as SBS's for that exact reason.
So, that just means you need to get a C&R 870
Once you have a legal SBS, I don't think there are any legal reasons why you couldn't attach it to a reg'd AW. I'm not even sure if it would be a problem hanging one under an OLL.
But in the end, the common feeling is that the masterkey setup is a waste of time and not worth it for use in a real CQB arena. But, we aren't concerned with that, its all about tacticool, and I think a masterkey definitely qualifies.