What experiences and skillsets should an experienced 'generalist' marketer look to really profile and play up when applying and interviewing for a fully dedicated PMM role?

First I think a generalist needs to learn about the core pillars of product marketing. I have a post I’ve written about the four pillars of product marketing. I would recommend that a generalist marketer figure out how their experience has taught them about each aspect of product marketing and have specific examples to make the tie. 

What PMM leaders are looking for in a candidate really comes down to 3 things.

• A deep understanding of the target audience. As a generalist marketer, you probably have used a variety of tools - Hubspot/Marketo, analytics tools, virtual event platforms, etc. You are more likely to get a PMM job at one of those companies because you literally are their target audience. 
• Great messaging/positioning/writing skills. Highlight web pages, blog posts, emails that showcase your skills. Include data to showcase the effectiveness. 
• Business acumen. If a PMM leader is going to take a chance on you, they need to feel confident that you will drive impact. Highlight the ways you, as a generalist, have had business impact.

At the risk of over-simplifying product marketing, you'll want to look for experiences that demonstrate some of the underlying skills to do three basic "jobs":

• Research: Find a situation where you've gathered the information yourself. Ideally this will include some first-party sources like customer interviews. This could be as simple a conversation at an event or social post comments. You'll also want to show that you can get information from different sources and make sense of it. A real-world example for a product marketer might be combining details from competitor reviews, win/loss interviews, and competitor job descriptions.
• Positioning: You need to demonstrate that you can understand a market, understand where your company/offering sits within it, and distill that into a succinct description. That's positioning in a nutshell, but you can find smaller examples all over the place. Ideally, your example here would flow from the Research proof-point above. When you get the part where you combined information from a few places, be sure to but a bow on what all that information ultimately means.
• Messaging: You need to demonstrate that you can create resonance. Describe a situation where you used some of that Research from the first bullet to build a deeper understanding of an audience, especially on an emotional or social level. To fully address the messaging skillset, be sure to describe how you used that understanding to improve something for a public audience. This could be Ad copy, tradeshow assets, etc.