I would imagine that dentists get taught the same amount about lupus as GPs (not a great deal but they have a basic understanding). One of my dentist's exams was on lupus, by chance. They also tend to know about meds in general, so I'd think they were much of a muchness between dentists and their knowledge of lupus.
But you'd be sure to get a dentist with more lupus knowledge if they work in a hospital where a lupus speciality clinic is, like St Thomas' in London. I don't know if you can get to see a hospital dentist, though.
If you want my Canmore Dental Clinic
details, PM me. He's in Middlesex.