*UPDATE* [2012-09-04 11:03PM UTC]
A fifth SystemUpdate has been deployed to all those in the Public Beta. Another eye-crosser this time ("145"->"154"), with the new version of the Xbox 360 dashboard/kernel now being 2.0.16154.0. Backups are working fine with LT+ v3.0 both offline and on LIVE, and the same goes for xk3y both off and online.
*UPDATE* [2012-09-11 06:31PM UTC]
A sixth update for the Xbox LIVE Update Public Beta program has just been released. This update brings the Xbox 360 dashboard/kernel up to version 2.0.16167.0. As with the previous updates so far, backups under LT+ are still working fine on LIVE and offline. The same applies for xk3y, which also continues to work without issue both on and offline.