I know that Louise and Gus has had petplan pay out for dentals before, although most dental work doesn't seem to be covered with most insurers as it is considered to be a normal, routine thing that needs doing, a bit like vaccinations (and more to the point, probably constitutes 80% of most bunny related problems so would be horribly costly to them!) By saying it's preventative work they're not saying that it's your fault, but many dental issues can be detected and dealt with before they cause actual illness, so they deem dental work as being something that prevents an illness rather than causes it :? My cats dentals have never been covered by their insurance either, and those are about £150 each.
The policies I've seen have said that dentals are covered if the rabbit has had a full vet dental check within the past 12 months and any work needed was done at the time.
It's one of those things where it's worth asking them if they'll pay up but not being surprised if they won't.