Não.
não se destina a ser usado até que a magia tenha realmente feito efeito. Considere o Sendic Advice Compendium sobre o tempo de Dispel Magic . Está bem claro que o RAI é que Dispel Magic não pode ser usado até que a mágica tenha realmente resolvido:
Can you ready dispel magic to stop another spell from taking effect?
The easiest way to stop a spell is to cast counterspell on its caster
while it’s being cast. If successful, counterspell interrupts the
other spell’s casting, and that spell fails to take effect.
Counterspell works against any spell, regardless of a spell’s casting
time or duration. With the Ready action, dispel magic can be cast in
response to another spell being cast, yet dispel magic can’t
substitute for counterspell. The main reason is that dispel magic
removes a spell that is already on a target, whether that target is a
creature, an object, or some other phenomenon. Dispel magic can’t
pre-dispel something. If a spell isn’t already present on a target,
dispel magic does nothing to that target. The best that a readied
dispel magic can do is dispel a spell immediately after it’s been cast
to prevent it from having any e ect after the action used to cast it.
For example, on your turn you could say something like this: “I ready
dispel magic, and if the high priest casts a spell on anyone, I cast
dispel magic on the target if the spell takes hold.” If the high
priest then cast hold person on your companion who fails the save
against it, you could unleash your readied dispel magic and end hold
person.