Has anyone had experience of this, either as a publisher, advertiser or end user? Any advertising platforms worth looking at? A quick look at AdSense ToS suggests that that isn't gonna fly.
This should be easy to test: create a new landing page, add a link to your emails ("10% off our pro package this month only!"), and track clickthroughs/referrals. If it does well, great. If not, you only spent a couple of hours on it.
i) There was a clear expectation when I signed up ii) You handle the ads; you're not giving my email address to anyone else iii) There's a clear way to get ads turned off (paid upgrade is fine) iv) Ads don't interfere or obscure with unsub links.
Be aware that I HTML email sucks and is evil. And email with images in is also pretty suck and evil. I'm aware that I lost that argument years ago, and so I can receive html email, and I can chose to view images.
Ads for non-related products would only seem to me to be detracting from your future business for a small payout now. My guess is it is a poor long term plan.