Make a paid bumping button, + free bump for every earned merit.
0.01 BTC per bump and no free bumps. Aside from buying theymos another yacht this also has the benefit of sheer simplicity. No matter who creates how many threads etc - anyone can bump it for a fee.
I like the idea of the owner having a "bump" button, and therefore completely eliminating paid bumping by any means. As an add on to this, I would suggest that each user can only bump a maximum of 1/3/5/whatever-number-you-like thread(s) they own per 24 hours, to stop the creation and bumping of mass dummy threads. Might be worth having a caveat that the owner has to at least be a member or have at least 10 earned merit or something similar to stop newbies from creating mass threads.
There are some other issues with this. E.g. the OP going on a vacation or just disappearing outright would mean the thread can't be bumped anymore.
Any paid option will just mean money is leading the forum.
It's already happening, what with all the bot farms etc. Making it "official" would reduce spam bumps, which is one of the goals here.
Although I would still prefer a merit-based approach.