I don't know how true that is what you read since most advertising networks now host a website where you go and rent space or sell space for advertising. While some advertising networks may bot sites to find potential partners these are advertising companies that wouldn't interest me at all.
Advertising networks care about the amount of visitors a site has and what your niche is about including things like bounce rate, using meta keywords will not display this information but say for example they use Google or Bing within the niche they are searching the chances are if your high on the search results then your be of interest of them so I'm not convinced that meta keywords is the way to go since they have many other ways of finding good partners.
If you are interested in selling or buying space for advertising I suggest you take a look at these advertising networks:
Buy and Sell Advertising Networks
- Advertiser Bay
- Project Wonderful
Even more information
- Successful strategies for selling ad space