Not the OP, but how I'd do it: for a v1 there could be some manual outreach and take their word for it, v2 hook into the APIs any ESP provides (probably 2/3 of the big players for this should have outbound reporting APIs), and then the v3 would be a custom pixel that newsletter owners would integrate to be "featured" on the platform.

v4 could then offer manually created ad units, and v5 could be self serve.