I have the impression that serious backstrokers find 200m quite long enough. Back crawl isn't quite as efficient as front crawl and so is more strenuous. I believe they do have long backstroke and breaststroke races for masters in Australia and of course you are free to race a freestyle using backstroke if you wish. I have seen a record set in 800m free by a backstroke swimmer (Willy van Rysel W90-94) and have also seen a very good 800m free swum by a male backstroker (not sure of the age group but in the sixties I think).
Some open water races have special categories for breaststrokers, but I don't think any have backstroke categories.