I appreciate that's an attempt at courtesy but it risks coming over as passive-aggresive. If someone said it to me, the polite phrasing wouldn't disguise the intention behind it and I would still feel uncomfortable refusing.

The original post is one of the rare etiquette questions to which there is a categorical answer: don't ask. Ever. If you genuinely need for a seat that's already occupied, ask the cc to help sort it out. Using a third party keeps it impersonal. A need is, say, a medical condition or having to sit next to a child or an incapacitated person. Wanting to talk business or hold hands is merely a preference not a need, and doesn't justify asking someone to give up their seat.