Based on my experience with cymbals, where it's less subjective than with trumpet where you introduce a lot more variables when you add the player (each player has a unique sound and embouchure, which plays a large roll in how a "trumpet" sounds and plays, where with cymbals, the results may vary a bit with technique, but not to the same degree) I would say that yes, hammering/peening by a master artisan craftsman would have a positive effect. With cymbals, the difference in sound between cymbals that are a.) machine hammered, (programmed pattern of hammering done through automation) b.) "hand hammered" (where it's a machine doing the actual hammering but it is being guided by hand) and c.) true hand hammering where the artisan is beating on the cymbal on a special anvil with an actual hammer in their hand is pretty noticible. It's light years between method 'a' and method 'c' - method 'b' gets it pretty close and probably creates cymbals that are consistently more similar between different cymbals of the same size and model, but they don't ever quite get to the same level of character that a cymbal hammered by a master cymbal smith possesses. Likewise, I think that a similar thing could be said of hammered trumpet bells - it just seems to me that it would bring out a unique resonant character with more body to the sound, and it would likely be much more repsonsive and resonant - the player wouldn't have to work as hard to play, but it might introduce some other sonic quirks - such is the nature of being unique.