I believe Jet2 lease an Air Tanker A333 for this purpose for the summer season only. I don't know the pattern but when I saw it on flightradar it was heading to Tenerife which is one of the longer flights in the short haul Spanish holiday markets.
The Jet2 a/c (G -VYGL) is MAN based and is indeed used on routes to the Canaries and Palma. It is currently just NW of Barca inbound from the latter.
They are however in the charter or package market rather than say easyjet capacity boosting with a larger frame and as you say not terribly common. I'm sure you're right that the economics has to be pretty marginal based on using an aircraft otherwise between long haul rotations.
Jet2 is an interesting case: it's a LCC which has become quite heavily involved in the package market. We have to assume that the seasonal use of a single A330 makes financial sense for them.
Thomas Cook again seem to use one of their TATL A333 but not sure, either way not many.
TUI I seem to recall have a dedicated short haul 788.
Similarly to Jet2 Thomas Cook have 2 Air Tanker A330s on lease but they seem to be used exclusively on their ever-growing TATL network from MAN. Thomas Cook do however have an A330 on lease from HiFly Malta which is being used on Canaries/Med flights.
TUI on the other hand, at least for the summer, are operating in a decidedly old-school manner with their 787s being used seemingly indiscriminately on both long-haul TATL routes and also Canaries/Med routes from a wide variety of regional airports (including DUB!) to an almost bewildering selection of European destinations. During the winter I suspect they will be concentrated on the long-haul runs including the likes of Goa and Phuket.
To get back on topic back in the day Cathay's Manchester flights used to route via a variety of European airports, definitely including Amsterdam and Paris but possibly also Frankfurt and Zurich, and I'm pretty sure they had full fifth freedom rights on these. Air India used to fly A310s from Mumbai via Delhi to Manchester via Rome but I'm not sure whether the latter was purely a technical stop.
Someone mentioned other TATL "extensions"; both United and Air India used to route their New York/Delhi and/or Mumbai flights via Heathrow. I suspect
that these flights had fifth freedom rights. Indeed the United flights once formed part of a round-the-world itinirary: did United hold fifth freedom rights for all legs?
On the Irish front at one time during the run-up to Christmas BA did send the occasional 747 to Belfast