For as much as I love my chocolate (Sig SAN stuff) and my peanut butter (HK Stuff), I don't know if I'd want them together. The regular B+T side folders have always been my side folder of choice (aside from the Chote K), I've never really wanted a side folder on a full size HK pattern rifle myself... and I imagine after it is all said and done a Sig 55X stock and adapter will more or less feel very similar to the existing B+T offerings, but weigh and cost more (if that is even possible with B+T stuff).
Whatever floats your goats, but my interest level honestly is zero. It is an interesting quirky rare thing, but I wouldn't pay money to own it. Kind of like that ACR stock adapter for the SCAR... Interesting idea, not my cup of tea. YMMV though.