I understand that it makes sense in many ways to have a discussion board or dev logs per title/product but it can be hard to manage especially if you’re an asset creator or tool developer with many projects.

Is there such a thing as a global discussion board or dev log?

If not, would it be against the rules to create a product page with no actual product but contains a discussion board that can be used for all products available?

No, there isn't a per-creator forum or blog, but you can create a placeholder project just for that, probably in the "Other / Everything else" category. Hope this helps.