AbigailII: you could think about categories instead of a rating, like normal, fast, very fast.
Fast would be one move a day (in average), very fast more than one move a day. Even living in a timezone of central China you could reach the veryfast category. I'm sure some really can join BK for a short quarter hour dayly only, I don't blame busy people for not making their moves here, in that case of course you wont reach a fast category. But someone who is here one or two hours almost dayly can easily reach the 'veryfast' category. How? You answer all your games once a day (makes you a fast player), now you have to hope that only one opponent moves back in the same session and answer that back too. From a certain amount of games (I guess 60, but certainly with 300) you'll be sure that this happens, in every timezone you can do this. And once you singed up for a tournament the system doesn't boot you, like you sing in for a tournament for players above 2000 but when it starts you might be below, that doesn't matter.