What are your takes on product marketing as a consultancy / outsourced capacity? Can PMMs only succeed when they're welded into the organizational backbone?

Product Marketers' main value add is their customer, market and product knowledge. It is also their ability to connect the dots between different parts of the company and its offerings. This takes time to build so it would be difficult to outsource. I wouldn't advise it on the long term.

However, an external PMM consultant could be useful in setting up the right tools, frameworks and methodologies in a less mature company and team. Temporarily helping to build foundations and scale a team as an example.