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Probably could get one of those cassettes with the dinner-plate 12th sprocket that leans out and just barely clears the spokes but fits on an 11-speed freehub body
Ahh no, the spacing between gears is different on those Eagle 12 speed cassettes. So you can't use those. You really need an 11 speed XD cassette, like the 10-42s of SRAM or 9-32s of 3T. And you add the 12th with an "adapter cog" like, but not limited to, these :
No it doesn't, they already have a 12 speed road freehub body called XDR and it's already on the SRAM 900 and Industry Nine CX hubs, notice the 10 speed and 11 speed spacer on the last one.
Also the Apex1 rear derailleur easily has the reach to clear that extra 12th sprocket. I'm guessing SRAM is gonna do the same as me somewhere Q4 2018 and call it "Red1 Eagle" and sell it for huge money. They really just have to replace one part on the current 11 speed shifters.