Xamarin have released information about how and when to upgrade to the new Unified 32/64 bit API's. Thanks to @redth, BTProgressHUD is already available as a Unified binary, but it was built against the current stable branch, 8.4, which is not compatible with the new alpha 8.6 API's.
So, as I can't (and shouldn't) release an binary built with the alpha channel version of Xamarin.iOS, I've put a one-off build up here. This is the same as what's on the component store right now (v1.14), just built against the 8.6 Unified build.
If you are still using the classic (old/normal) APIs, you don't need to do anything - well, you do, you need to port to Unified before 1 Feb 2015, but Xamarin has your back for that. This build is just for people using the Alpha channel who have got X.iOS 8.6 recently.
To use it, just remove BTProgressHUD's component from your project, and add this DLL in. Thats it. Once the proper component comes out, just remove the DLL and add the component back in.
Happy HUDing, and thanks to Gerry High for pointing this out to me.