To avoid derailing PEP 621, I’ve made a proposal here for taking the idea of dynamic and adding it to the core metadata for use in sdists.
If we can agree on that, I’m fine with abandoning the idea of backends writing to pyproject.toml. (If we can’t agree on that proposal, I may have to abandon backends writing to pyproject.toml anyway, but there’s less risk of PEP 621 being caught in the fallout if we can deal with sdist metadata separately).