Any elegant way to 'override' a No Bulk Mail setting for a single mailing?
Ignoring the no bulk email flag isn't, and shouldn't be an easy process, so what you describe seems to be the "less cumbersome" possible way indeed.

However, it seems to point out to a related issue: these 400 people shouldn't have opted out, but rather it seems should have been unsubscribed from the "regular" newsletter group and kept into the "VIP statutory meeting announcement only" group.

In theory, you can't really reach out to people that have opted out -by definition- but given your specific situation, I would contact them to tell them that you need to be able to mail them, and explain the new "statutory meeting" vs. "newsletter" situation.