While I initially was leaning towards a separate field, it makes more sense to me to reuse the License field.
Yes, it would be hard to discern from free text to a license expression with a typo, but this would still be possibly in many cases (say you made in a typo in a license id and not in a “and/or/with” keyword).
There are also several benefits besides this:there is immediate backward compatibility
And this much simpler to understand for users and authors. There is now a single license metadata field.
We have first a soft validation and warning and later (say in about one year from the time we have this adopted by major tools) we start doing a stricter validation, possibly rejecting incorrect things in pypi and other publishing tools.
I foresee a lot of confusion if we start having not two but three license metadata fields for a transition period.
That said, if there is a consensus that this is a better approach I will update the draft PEP accordingly