Isn't there already an OS X app for that:
https://www.mobileread.com/forums/sho...d.php?t=204157
And, last time I checked Sparkle was tied to XCode, which makes it a non-starter for calibre. Also why do you need an xz, IIRC Sparkle works fine with dmg. There is already an RSS feed for calibre releases (link is at the bottom of the calibre website). If it needs any extra metadata in its entries,let me know and I will be happy to add it.