Warning - this is a somewhat speculative answer, I haven't actually checked these methods for fresh plum vs prune taste, just logic'ed that they might work - especially given Mark A's answer about the boiling point of the relative flavor compounds.
A while ago, I saw a method of preserving strawberries that talked about the flavor differences from fresh to cooked preserves (jams and such). I don't know if the method used there will work for plums, or if the results will be similar using plums instead of strawberries - but it might be worth trying and at worst, might suggest a place to start looking for similar "uncooked" preserves specifically targeted at plums.
So this particular historical preservation method used unblemished fresh strawberries, cut up, sugar, and chopped up raisins - in a ratio of three parts (by weight) strawberries, to four parts sugar, to one part raisins. The raisins are used as a sort of preservative, and the recipe makes a kind of sweet pickled strawberry - they layered the sugar, fresh strawberries, and raisins in a covered jar, stored in a cool place for up to several months (for safety reasons, refrigerator would be wise nowadays) and over time the strawberries released enough liquid to cover the fruit with a thick syrup. The strawberries ended up a bit tougher and more dense (partly dehydrated, after all) but the flavor profile was apparently closer to "fresh" then "cooked" strawberries.
So, while I'm not sure of sugar and raisins will preserve your plums flavor in quite the same way, you can certainly look for sweet plum pickle recipes, or look into storing them in alcohol, or other sorts of uncooked preserves, and you can store these in the fridge or freezer if you want to be careful. The flavor will be more different the more other things you add, but you might like them anyway.
Another possibility might be to look into candying or crystallizing or glaceeing your plums - or making a mostarda, if you want, with the addition of mustard oil or powder - prick them all over, keep them in concentrated sugar syrups, and keep filtering out, reducing and replacing the syrup and letting the plums soak - I should note you want the old, slow method of soaking and reducing just the syrup and soaking some more, rather than the quicker method of repeated simmering if you don't want the plums to taste cooked. Over time, the water will leech out and be replaced by sugar (and be reduced out of the syrup), leaving you with a batch of shelf-stable candied plums. Again, since the plums aren't cooked, you should get more of the fresh plum flavor than the prune flavor. Find a good recipe for this, though, it takes a known recipe with good ratios to be safe leaving sugar syrup and fruit at room temperature for weeks or more.