It's not a pipe dream.

Startups convince investors to give them money for all sorts of reasons and with all sorts of potential ROI. Advertisers at this early stage could be regarded as investors of a kind. If they believe in your project and your game plan, then they might want

(A) to support you
(B) to lock down cheap long-term rates for their own advertising

By supporting you, they help build the audience that gives them exposure. And if they're able to guarantee a lower advertising rate now versus later, then they may see the payoff as worth the risk.

Very similar to pitching a startup to investors. It's only a pipe dream if your rates are too high, you can't identify the right prospects, or you can't present the case effectively. But if your early-bird special is a bargain and if you can find the right people to talk to ... and convince them, then go for it.

Perhaps you could sell them X months but start the clock only after monthly traffic reaches Y. That way, they'd get the preceding months for free, and they'd see that you're properly incentivized to build traffic early.