Are you talking about this rule:
24. Advertisements (including signatures within the post area) in posts aren't allowed unless the post is in a thread you started and is really substantial and useful.
And:
Ads are typically not allowed in posts (outside of the signature area) because they are annoying and off-topic. It is especially disallowed to put ads or signatures at the bottom of all of your posts. Except for traditional valedictions, which are tolerated but discouraged, signatures are for the signature area only.
However, if you are using the forum as a publishing platform to host something really substantial and useful, selling ads in that substantial work is allowed. To be eligible for this, your post must be in a topic that you started, and your post must be substantial and long enough to make the ad seem entirely insignificant. If in doubt, ask me.
I don't think a bounty would quality as "something really substantial and useful" but you should probably ask the Benevolent Dictator to clarify.
Thanks, bud. Reads to me like the key phrase is "typically not allowed", thus not forbidden. The gist is that the OP MUST have substance of sorts so to encourage discussion.
Re utilizing such as a bounty campaign, I envision the starter of the thread being rewarded per some metrics incorporating page, post and view counts.
Further thinking out loud, the clause placed at the top of the OP won't be added till after 20 posts/2 pages is obtained. The clause could be --> This thread now sponsored by <linked-to entity name here>.