Any suggestions on how to build and validate market driven messaging, versus the easier and more prevalent product/feature centric messaging?

For guidance on how to build messaging, see my answer to, What are good messaging framework resources that you use?. In terms of how to validate it, I’d recommend a combination of the following tactics:

• Talk to your customers: Invest time talking to and learning from your prospective and existing customers. Specifically, listen to how they describe the problems they have, the pain points they experience, and what a better world would look like for them (in their minds).
• Partner with your sales team: Your sales team are on the frontlines and spend a lot of time talking to prospective and existing customers understanding their needs. Work with them to test your messaging on calls and in conversations. It’s amazing and eye-opening to simply listen in on a sales call, or watch a live chat conversation unfold, and hear/see what resonates and what doesn’t. It's so, so valuable.
• Conduct user tests: We're fortunate enough to have a dedicated Research team that we can partner with to test our messaging and positioning, particularly for new product and feature launches, to validate our own assumptions and ensure the story we are telling resonates and makes sense.
• Run A/B tests: To the extent you can (you need volume), run A/B tests on your primary marketing assets (e.g. ads, landing pages) to see which messages drive the best conversion and LTV throughout the funnel.

Similar to the question above on uncovering buyer trends, I’d suggest starting broad and then going narrow to develop market driven messaging. Start with a few key insights about your target market and their pain points. I leverage personas for this. If you don’t have them, the work should start there to really understand your customer’s needs and concerns.

From there, I would build some options and do high-level testing with focus groups or customer panels. I’ve also done this via quantitative surveys where you ask prospects to react and rate how they resonate to different messages. You can also always test in market if you have the capability. For instance a demand test to run ads with message x and measure performance against your usual slate of market messages. If it fairs well, you can go a level deeper and create a test landing page experience to see how well the messaging converts.

Talk to your customers, write 3 messaging options and ask them wich one they think that explain better what you want to say (or better connect with their pain/or make you different).

We used SurveyMonkey combined with some challenges in our community platform to validate. And sometimes A/B testing with Ads. 

Product positioning/value propositions/messages are developed for people (personas) in markets. The first step is to identify the personas that influence a buying decision for your products. When you understand that, you can dive into the problems each persona is experiencing and develop a value prop for each. A message for a technical buyer won't resonate for an economic buyer, as you point out. Trying to create one message that resonates with both may be impossible (and will be too generic). 

Building on what Matt said, once you know the people you can engage in conversations to understand them better. Hopefully, you are doing this long before the product is ready. 

Example: breakfast cereal for children. They eat it but they don't buy it. You need one message for the kids and one message for the parents. You want the kids to annoy the hell out of their parents so they buy it, and like it so much they tell their friends. You want the parent to believe they are making a good dietary decision. 

Are you talking about messaging specifically (precise wording) or a more general positioning? If it's the former, then I think Lia is correct. For the latter it takes some pretty heavy primary research. The way I do it is by spending time with a client's customers, prospects, or others who would be representative of such, asking questions. Lots of questions. The key is to ask questions about anything but the product/product category. Learn what preoccupies their thoughts, then figure out how your product relates to that.