I may be missing something, but why not just set up a separate appcast? Is there any reason why your v1 users _would_ ever want to find out about v2 releases? (Aside from a general sort of "hey, v2 is available!" announcement, for which Sparkle doesn't quite seem like the right venue to me.)
With a separate appcast, you could even continue releasing minor v1 bugfixes if necessary, without worrying about breaking things for v2. - Sean
