Perhaps some automation might be helpful?
Could the solved plugin automatically chase the author (via PM) to mark a post as the solution if the topic is responded to and there hasn’t, for example, been another post for a period of time (and no marked solution)?
That chase could include instructions on how to do this?
That chase could also appear below the topic for the author after a set number of replies?
Some statistics on how long it usually takes for a marked solution to appear in terms of time and number of posts might help tune an automated solution.