How do you ensure that product messaging and positioning are effectively communicated in demand generation campaigns?

Messaging framework all the way. Designate a 'source of truth' messaging doc that everyone at the company (not just demand gen) refers to, and therefore writes/presents derivatives of. This source of truth can be as simple as a starred google doc, or a published confluence page, notion page, pinned/bookmarked in Slack or MS Teams, etc.

At smaller companies I've seen demand gen marketers request product marketing to write emails, ad copy, and campaign briefs in order to start a campaign. IMO those are not constructive (or scalable) in growing into a world-class marketing team. If a messaging framework is in place Demand Gen marketers have the opportunity to take a stab and ask the owner for input and feedback. It's amazing how far effort and thoughtfulness goes into building trust and facilitating stronger relationships.