Wade Stuteville made a 6 dasher barrel for my AI back in 2015, but I cheated and used it on my AIAE as I knew I could easily get it to run and feed from the single stack AICS magazines used by the AE... I tested feeding with dasher dummy rounds feeding into a 6.5cm barrel as proof of concept before I gave him the go ahead on the barrel. Pretty sure someone else had tried BR or dasher in an AI before I did because I remember asking about trying BR or dasher around 2014 and was warned primer cratering might be an issue. The issue was most AIs at that time had large firing pins which didn't play nice with small rifle primers, luckily Wade was one of the only ones offering bolt bushing and turning down firing pins for AIs at that time.
This was before anyone was offering BR/dasher spacers and followers for the double stack AW mags. (Josh, I did use your AICS mag spacers in my 6 dasher AE and they worked great.)
As far as weird calibers in an AI, for a couple years I had one of Gilbert and Enriques 223 conversions in my AX...not very common. I think they also did a one off 6 Valkyrie conversion for Frank.
AI themselves offered some oddball cartridges in the very rare AW Varminter in the late 90s... they offered 223, 22br, 22-250, 22 middlested, 243, 6br, and possibly 22ppc and 6ppc, so I suppose AI themselves were likely the first to offer AI barrels in the BR family-- although in very, very limited quantities.