great idea, bliss. i've tried that with dried mushrooms, with mixed results. i'll have to try dehydrated tomatoes. i wonder if well chopped sun-dried tomatoes would work...hmmm.
i agree with andy; paste will change the flavor a little. it depends on the ratio of how much you're adding to how much sauce, though. paste tends to be sweet, which usually isn't a problem.
reducing by a slow simmer, regularly stirring to prevent burning and aid in evaporation is the way to go. but as gadzooks mentioned, if you're in a pinch, you can use cornstarch or arrowroot slurry. it was suggested to me by the chef at a local italian restaurant, when i asked him about his sauce. he uses fresh tomatoes in a lot of his sauces. when i'd mentioned that my garden tomatoes one year made a runny sauce that takes forever to cook down, he clued me in about using a thickening agent. worst comes to worst, he even suggested using a little flour, but a little goes a long way, so be prudent about it.
A true lover of nature does not despair now that his mistress has turned a colder cheek.