Yes I did consider the SS cartridge since the rifle will be a single shot but like the 22 Mag I wanted this to be a switch barrel rifle with a second barrel in 218 Bee, however considering the even rarer availability of Bee brass that might not be such a good idea. Still with enough effort and the willingness to share the same headstamp for different calibers it is doable even if not really practical.
Also you are certainly right about the brass, the SS brass not only seems even easier to find but quite a bit cheaper to boot!