Ask enwiabe if you can make a sub-forum within English & ESL perhaps. This will inevitably get quite spammy so I think it's for the greater good.
As for the markers, I think it might be a problem. As AN gets larger and larger, we're going to inevitably get many more members wanting to join this group next year. However, the number of markers is not going to increase. I'm prepared to mark a few essays a month, but I assume eventually the number of 'qualified' markers is not going to be enough. My solution is that I think at some stage, you guys will need to be able to mark your own essays. This is a skill that you will develop yourself because the best way to learn English is to mark it yourself. The only reason why I am now able to mark essays is because I marked my own; it's not like I went to essay marking school or something. So I propose that eventually, especially for text response, you guys will need to mark each other's essays. This works out well since most of the qualified markers haven't done the texts you guys have done. This way, you guys can also read each other's essays and learn from them. It's a process of mutual benefit really. Context and LA will probably keep the qualified markers busy enough as is, then yeh, perhaps leave text response to be marked by one another? Perhaps the qualified markers can just briefly skim over them and ensure the expression and structure is fine, but I think the bulk of the work will rely on you guys marking each other's ideas and such.