@daan, this is a great piece to read! Are you familiar with #promo-Steem. It is a tag where people doing promotional related content for Steem post to. There is a multi proof of brain guild up voting it from the account @steem-ambassador. Maybe this can help a bit with marketing efforts.
Certainly would be great to coordinate together on marketing initiatives! Have u seen the 60 seconds of Steem videos that surfer Marley has been doing recently?
Oracle-d has a large team of writers who help blockchain businesses and projects create content on and off the Steem blockchain.
Oracle d has done a lot of crowd marketing initiative for Steem, but as you suggest, we need a place to pool these resources with other communities in order to have more affect.
Database of free resources is a great idea. About a year ago www.promo-Steem.com put a media page together, but it is far from exhaustive. As we build out the Steem business alliance web site, we may be able to displace certain curated content as down loadable for public use for marketing materials

Very interested in talking to you further on these things, and your ideas!