I don't think any end users will want them, and I don't think they're going to convert very well. But I don't think it would feel obtrusive if properly separate

I don't think any end users will want them, and I don't think they're going to convert very well. But I don't think it would feel obtrusive if properly separated (eg. as an email footer or header). 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.