My understanding is that Defense Tokens and Accuracy results are both spent before resolving damage, which should mean that you can spend them to trigger abilities. Is there any part of this that I am missing to prevent this use?

The only time you couldn't pass through each step of the attack process and therefore spend tokens if you wanted would be if your opponent was unable to gather dice and thus end up with the attack being cancelled.

After that the only thing I can think of would be the situation where your opponent rolled as may accuracies as you had tokens and could prevent you using any.

IF the opponent gathers dice, finds out they're all out of range/obstructed, the attack is immediately cancelled... Boom, over.

HOWEVER.

Lets say he does attack, and rolls nothing but Blanks...

You still get an opportunity to select and spend Defense tokens if you want... (Such as getting them ready for discard for Devestator or somewhat).

So, let us look at the absurd situation...

An ISD-2 attacks with its front arc, and rolls 8 ACC results...

You still proceed through.

You ask your opponent (or wait for his statement) which ACC results are being spent to lock out defence tokens... If they don't say any ... Then you are free to spend defence tokens... Even if they would have no effect...

You can spend a Redirect Defence Token even if a ship has no shields........ It just, obviously, doesn't have any effect...

And in the core set of rules, there is no incentive or use for doing that... So it must be upgrade related to want to do so - and indeed, those upgrades are very few and far between right now (Devestator being one).