Keep bundles together when splitting orders

We have several clients who sell bundles (e.g. gift packs) and customers can purchase other items. Rather than holding the entire order until stock arrives we are requested to split the order and send what we can.

However - if part of a bundle is in stock and part isn't - the bundle is also split and items released for packing.

We want to keep the entire bundle contents back until all the stock is available.