...it would also be possible to announce every single bounty on bitcointalk. Advantage would be: More attention. Disadvantage might be that people at bitcointalk don't like it when their forum is "hijacked". What do you think?
I think we could ask them for advertising space and thus pay for the use of their services?
you mean pay the bitcointalk forum for a child board? 
Pay them for Ads that link to this forum rather than Ads that link to Butterfly Labs or other mining vendors?
It might be worth trying. Generally I think such advertisement only gets your attention if you know the brand anyway. I perceive most of the ads just as colored bars. But it could work with I3. The words Bitshare and Bounty in a blueish color scheme should work. Linking to a forum page where the intentions (getting bounties done, finding talent) and the referral bounty is explained. Also a good place to present I3 as moving forward, well funded etc. It shouldn't cost much anyway...
What could be done anyway is to post each bounty as an answer to my topic. I would relinquish the ref. bounties of course

This way the post stays up always... Do topics get pushed also by editing? They could be edited to push...
I addition I would post some of the most important and best paid bounties on Bitcoin Forum > Bitcoin > Project Development to attract attention and also refer to other bounties at the bottom of the post. The text would just have to be copy pasted from the bitshares forum...