The DMG is pretty explicit about not allowing ready actions outside of combat, but it's much less explicit about when combat should start. I think it's perfectly fair to say that once two opponents become aware of each other, either side can choose to initiate combat by taking any combat action, including readying an action. This state should persist until both sides cease to be aware of the other, or both sides stand down.
In fact the DMG gives an example much like this, where two parties become aware of each other but cannot immediately interact (pg23). In this case, actions start tracking by rounds, and each side can do whatever they want to prepare until they come into contact, at which point initiative is rolled. The question is, does combat begin when initiative is rolled, or when the parties become aware? I'm not aware of anything explicit in the RAW that says it can't be the latter.