If I want to implement a templated container supporting methods like find, can I cache the script function like opEquals within the template callback.

I've checked the source of official script array add-on. Its template callback only contains logic for validating subtype. The logic for caching methods for comparing subtype is done elsewhere.

With the current implementation it should be OK to create the cache from within the template callback itself. But the callback is not really meant for that purpose, and I cannot guarantee that it will always continue to be valid with future releases.