Just about any system for an auxiliary will work fine, the critical concepts are to use a long hose, donate the primary, which is on the long hose, and be diligent and responsible in practicing deployment of you auxiliary and dive control whilst using your auxiliary. That said, I have a set of concerns over the necklaced axillary. If the necklace is secure enough for one to have no concern of it coming loose then there maybe a concern of the damn thing choking you. If it is loose enough to assure you that it can not choke you then there is a concern of where it will actually be when you really need it. This is a problem in design optimization upon who's outcome I'm not sure I want to stake my life. But that's just me, and I'm sure that my best answers create issues that would make other divers of equal skill and experience equally uncomfortable. Like with most things, there are trade offs, you pays your money and takes your chances. Do what seems best and most comfortable to you. Most of all drill and practice 'your emergency procedures 'till you've experienced all the things that can go wrong (and some that can't!).