I believe what I have read on here is that you should run an 11 speed cassette with a 1mm spacer at the rear and leaving off the smallest cog. You'd have to use the next size up lockring as well. Say you had an 11-28 11 speed cassette, you'd put on the spacer, slide on the cassette, remove the 11T cog and use a lockring for the 12T.

You can use your wheel, but you will need to omit one of the cogs from the 11 speed cassette. I do it by getting rid of the 16t (assuming you're running the standard 11-25, I think you could also pull the 15 or 14, but I haven't tried it that way so I can't vouch for it). Then take the extra plastic spacer you have after omitting the 16-tooth and use it behind the cassette on your 10 speed wheel (i.e. the very first thing that goes on during installation is that spacer). The rest of the cassette goes on normally (minus the missing 16t of course). Now you have 10 cogs at the correct 11 speed spacing and it all fits on the 10 speed freehub. You may need to fiddle around with the "barrel adjuster" (as an aside, what the hell are we calling this in the Di2 age?) for a bit to get it shifting smoothly. You will also want to adjust your limit screws such that you cannot throw the chain off of the back side of the 25.

I've been racing on this setup all season and it works fine. The shift from the 15 to the 17 is a bit shitty, but it goes (it just sounds like hell and takes a split second longer than any other shift).

There are various options:
- take an 11sp cassette, remove 1 cog and put it on the 10sp freewheel, you now have 10speeds with 11sp spacing
- put a Campy 11sp freehub on the wheel and an 11sp cassette (Campy and Shimano have same 11sp spacing)
- get a Dremel and grind down the freehub so that an 11sp cassette will drop down

http://forum.slowtwitch.com/...%20cassette;#5150065
http://forum.slowtwitch.com/...011%20speed;#5198278
http://forum.slowtwitch.com/...011%20speed;#5183524
http://forum.slowtwitch.com/...011%20speed;#5082650