One way I could see it working would be to have a dedicated diagram from Heaton each day - e.g. the 07:35 Newcastle to Southampton (arr 13:18) - departs Southampton at 13:47 and would be back on Tyneside about eight at night.
That way, you have a depot that already operates 180s, you could run a ten coach unit each day to free up a Voyager (to double up another diagram) - you'd have to ensure that it didn't stray from this one diagram but XC seem to be able to keep the HSTs on fixed diagrams so maybe there's something to be said for using the Heaton knowledge of 180s and giving XC a daily return journey operated by a pair of them? Say three units to ensure that there are two fit for service each day, on a rotational basis?