I'm attempting to do the same (not using UICKeyChainStore) and running into the same issue. My entitlements file etc. are configured correctly (I can use it with another iOS app), but when I try and access an iCloud Keychain item (query includes kSecAttrSynchronizable == kSecAttrSynchronizableAny), I don't get any result. I can see that the item is in iCloud (can verify in Keychain Access on my Mac), but no dice.
Sometime telling: There's no prompt for iCloud Keychain setup initially, and under Settings > Accounts > iCloud, it's very barebones (photo settings, etc.), and where you'd traditionally have a row to manage iCloud Keychain, it is not there.
It'd be nice to know whether or not it'll be there by GM, or if is not planned for this GM.