eTapestry support has confirmed that currently there is no way to have an additional donation field when someone is purchasing tickets. This is a very common tool used in most event registration platforms (eventbrite, brownpapertickets, etc.). For the past three years our organization has used an outside event management platform specifically because eTapestry doesn't have a way to configure donations into event DIY forms. This seems like a fairly streamlined fix, and I would love for eTapestry program staff to consider it!
And, just in case a support staff responds with alternative solutions ... no, nobody wants to create a separate hyperlink in their event registration form to a separate donation page (which is what the guidance is so far).