We have nice shiny expert badges right now, which are based on the number of upvotes received in a particular tag. While that's all well and good, it does tend to bias the recipients towards those who make popular answers, not necessarily those who solve the problems of the asker. As a case in point, see the (in)famous recipients of the subjective expert badges.
Why not have another set of badges for those with many accepted answers in particular tags? This would reward people who go out of their way to help people with obscure, not-so-popular problems and are able to solve them and get the acceptance checkmark, even though the only upvote they ever get is the one from the OP.
The threshhold numbers could be lower, of course, since any given answer can only ever earn 1 acceptance.
Alternatively, I suppose you could count the acceptance vote as an additional upvote for purposes of the expert badges, but I think that could conflate the issues here and diminish the value of the expert badges already awarded.