That's sure a cute contraption. I have a couple of nipple adapters (Fusil V's, made years ago) that allow you to use SR primers on a caplock - also have one that uses the 209 primers (Mag Spark). It seems to me that the flash from the 209's is hotter than from a small rifle primer -frankly, I see no advantage at all to using SR primers when you can use 209's.......

(BTW, such a device is pictured and described in Ned Roberts' book, The ML Caplock Rifle. These "primer nipples" were in use as far back as the 19th Century!)