Bugger!
My sleuthing reveals that the 11 speed Shimano HG freehubs are backwards compatible with 10 speed cassettes, with the use of a spacer (spacer goes on first, then the cassette).
Meanwhile the previous generation Shimano HG 10 speed freehubs are compatible with some “special” 11 speed road cassettes that have a concave 11th (biggest) cog carrier, like the HG700. They are also compatible with all Shimano 11 speed MTB cassettes (because they have the concave carrier). Unfortunately every cassette I’ve been able to find has two-tooth jumps at minimum… there’s no closely spaced “special” 11 speed cassettes that will work on the 10-speed freehubs as far as I’ve been able to find.