What is (or are) your exerted tomatoes (or, ones that you know of), that have the shortest season - i.e. that would be most suited for growing outdoors in colder/cloudy climates - and which are or have been available? I suppose if someone were to want to either make one of the promiscuous landraces, or some wild crosses, shorter season, then the earliest well exserted tomatoes would presumably be ideal breeding partners?
Or… I just had a thought (and my thought is mainly around choosing a partner for wilds and wild hybrids but I guess could also apply to already existing landraces) - perhaps another route could be to ignore exsertion and taste and focus only on size and earliness. The earliest reasonably sized tomato one could find, even if it is totally bland! And rely on the wild genetics for the flavour and exsertion, while trying to bring in as much earliness and size increase into the wild mix as possible! Would that be a good plan perhaps?
(Edit: Oh, come to think of it, I suppose that is what Joseph was doing! Silly me! Though I wonder, perhaps we can find earlier still, as it seems the promiscuous tomatoes struggle as they go North…)