The only thing I have found is the Uncle Mike's swivel set that requires you to drill your existing magazine cap. Really nothing to it, but it is not exactly what you (and I) have been looking for.
In the end I attached a "Swiftach" sling to my BPS. Rather than a swivel stud at the front of the cap the system uses a wafer that is sandwiched between the cap and the magazine tube ring on the barrel. The "wafer" is cloth of some sort and needed to be opened up for 10 gauge magazine tube but has given me good service for 5 years or so.
