What is your opinion of this email marketing idea?

These are loosely called PS clicks...as in the post script for a letter except in email form.

FYI, split test this extensively because it's already known and pretty much verified that these clicks perform less well than regular bodied email clicks.

You're essentially pulling attention away from the main body text by giving the subscriber more things to look at.

You're better off testing CPA offers or PPL offers with PS traffic since a solo ad run with PS traffic will have very low opt-in percentage.

Second this.

And put yourself in your readers' shoes. Honestly.

Take off your marketer/content writer/business owner/affiliate hats and think about how you read emails.

Do you care who "sponsors" the sender's email?

Are you providing that much entertainment via email and content that it even merits a "sponsor"?

Or will you sever goodwill with people who see the blatant pitching and the attempt at disguising it?

Purely personal preference, but when someone's selling me an affiliate offer, the ones I click on/buy into most are the ones where they provide the link and simply inform me they get a little something extra when I buy through their link. But first, they provided OUTSTANDING value before they asked anything of me.

So...sounds like you have some tough questions to ask yourself. Are you merely promoting someone, or are you trying to get affiliate sales through these "sponsorships"? Will these "sponsorships" cost you readers or will they increase the value of your emails in their eyes? Is this good for your customers or good for you, and which one of those matters to you more?

And I keep putting "sponsor" in quotes because it's unclear to me whether this is a two-way agreement, or if you've just decided unilaterally to use someone else's name to imply they support your efforts. I'd be wary of making a one-sided decision using someone else's name if I hadn't gotten permission first.