You may want to check with a Luthier builders supply company for just such a plane. I vaguely remember a guitar maker using one on YouTube.I have a Dremel with a pretty small round over bit. Not sure of it's size but it's tiny. It doesn't have a bearing but with a straight edge and a jig to maintain balance and depth it might work.
Another alternative is a an edge plane with a custom blade. I think there have been posts on the Forum about that a while ago. You can search the archives.