You can locally anneal brass/bronze and re-temper. This is what is done when repairing dents in horns. Let's be clear, tempering with cymbal materials is done through physical hammering/work hardening (spinning in the case of horn bells). It is not like steel, where tempering can be accomplished through various heat treating/quenching methods.
Locally annealling will change the temper in the material and will be difficult to return that localized portion to the exact temper it had beforehand...especially since you must be careful to hammer only that annealed area which is difficult to do.
I have no doubt that heavily hammered cymbals go through a series of annealling and retempering during the production process to avoid cracking and impart the desired final sound. However the final product is clearly at a certain level of temper through hammering...far from an annealed hardness. So the final process is hammering.
I have also removed dents in B8 using the wood form technique with good success. B20 is harder and crystal shape is different so it has propensity to crack if you whack it too hard.
Here's a nice analogy (that I can't take credit for) that might help understand what happens in brass/bronze materials throught the annealing/work hardening process....
Brass is an alloy with a molecular bonding of copper and zinc. Other alloys have similar
comparisons. The molecules of the alloy bond in a crystal structure that form a grain
pattern in the brass. Kind of like wood grain, but at a microscopic level. The crystals have
random shapes. Each crystal finds it's own space next to it's neighbor crystals in the
structure. When brass is formed or worked in a specific direction, the crystals start to
change shape and they start overlapping each other in the same direction that they are
worked..they start laying down. You think of them like Dominos standing up next to each
other. When the brass is worked, it is like the Dominos being knocked down..one against
the other. When it is all over..the entire row is laying down in the same direction. If the
brass is worked in a different direction, the brass crystals do not want to get up and the
brass may form cracks. It is like you take your hand and mess up the Dominos. When the brass is annealed, the crystals all get into their own space again..or the Dominos are
lined up again all standing upright. Excuse the analogy to Dominos, and brass crystals
are not as tidy looking as Dominos, but that is the best visual analogy.