Nice initiative, please consider adopting IAB standard ad sizes like 300x250, 728x90, 468x60, etc so you can access to more remnant inventory.
Right now many ad companies use Greenpeace or Red Cross for serving something when they run out of ads. It shouldn't be hard to get some impressions from them, but the standard ad sizes will be a common requirement.
I get the impression this is more geared towards sites that would run the Deck, Fusion, et al. In other words, sites that tend to have very limited advertising and don't subscribe to the IAB's definition of size standards.
Right now many ad companies use Greenpeace or Red Cross for serving something when they run out of ads. It shouldn't be hard to get some impressions from them, but the standard ad sizes will be a common requirement.
Good luck!