How to prevent keychain passwords being seen with iPhone passcode?

Solution 1:

I hate to reference unreleased software, but quite frankly the solution may be for you to enroll now in the iOS 9 public beta or simply wait for it's official Fall release. iOS 9 (at least in its current state) gives the option to set a custom alphanumeric password in place of the familiar 4-digit pin.

So, if you have hardware with touch ID capability you could use that for your everyday login and reserve a more complex passcode (even the same as your iCloud one, like you mention) for protecting your keychain in case of theft, etc.

Solution 2:

Simply use an alphanumeric passcode.