[burnished]

Yes, I probably should have omitted the example of the OP because I'm really more interested in the principle than the specific implementation.

My suspicion is that you could probably give verbal or informally written or offered license and that it would be valid from a legal perspective. I'm basing that off the existence of verbal contracts and how there is no process for licenses but instead a very practical consensus on the best way to communicate intent.

EDIT: In a sibling comment I verified this suspicion! Includes a couple links to short articles.