An IP laundering service certainly sounds like a potential startup opportunity. Certainly I'd have paid for a proven good IP in the past before I developed my current solution.
...as disconnected from "email marketing services" as possible, please, because IME gmail is wise to those and files email associated with them directly in the trash regardless of all other concerns.
I suspect the reason SES is an exception is because it is very widely used for things like e-tickets, transaction confirmations and so on, and also goes to a nonzero amount of trouble to dissuade marketers rather than having them as the main customers.
Email marketing services provide a similar feature called IP warm-up which does the same thing but over a shorter timeline.