Today I feel like I rediscovered ancient wisdom! Probably some (or all) of you may know what I'm about to tell but I didn't and it came as a very very pleasant surprise!
Ok, the "discovery".
It is possible (at least at SP23 where I tested it) to use dynamically template in data exchange!
Yes, you can overwrite the predefined template used by the import mapping, just by adding "Template" as attribute (as reference to name)
(see the attached document)
I'm so happy!
Btw, using this method you can alter the template (without however altering the values - though I haven't tested it thoroughly) of existing items. If you wanted ever to get rid of a template without deleting items that used it, well, that's the method!