How involved should the PMM function on Brand strategy? Where/how can Product Marketing contribute best on this front to really drive impact? How can we measure it?

There are 3 main areas where I've seen product marketing get involved in brand strategy:

1) Brand category definition: (see my other answer on how PMM & Brand would collaborate on category creation/evaluation)

2) Brand/product architecture: When you have a portfolio of offerings and multiple brands, the need will arise to revisit/simplify the brand architecture. We have experienced this when we've built new solutions, acquired companies, etc. Product marketing can bridge the gap between brand experience and product experience, making recommednations for where a product should live in the brand ecosystem based on how the product is built/integrated into other products in your portfolio or even overlap in buyer personas.

3) Brand value messaging: PMM can contribute to the creation of brand-level value propositions as they ultimately need to tie back to concrete product value.