Adverts by donation rather than fixed price?

I'd love to find a way to offer ad space on my website for DONATION rather than a fixed price. Does anyone have any ideas how we can pull this off?

Preferably, ad management would be automated in order to keep it low-maintenance. I have my eye on OIO Publisher, but they do not have the ability to accept donations -- only fixed pricing per ad.

How can I address this, perhaps using a creative combination of plugins?

Thanks in advance for your brainpower.

Aimee