Fine-Grained user permissions

I am wondering if the Ad Module could possibly allow more specific creation permissions.

My requirement is to allow a User to create X number of ads in a specific format, based upon their role.

For example "User Role Level 1" may create one Ad of format "Popover Landscape". Popover Landscape is afforded 10000 impressions and then becomes unpublished.

To extend the example, "User Role Level 2" may create one Ad of format "Popover Landscape" and one of "Skyscraper". Popover Landscape is afforded 10000 impressions and then becomes unpublished. Skyscraper is afforded 1000 impressions.

I know that this can easily be achieved with Custom Content Types but my main issue there is that I can't figure out how to define inventory. All automated expiry solutions seem to be calendar based.

Please forgive if this is a naive request and thanks for a great module.

:) S.