Next step is to draw up some draft "standard" critique requests (no critique, no line-by-lines, anything you like, etc.) for feedback and then amend the fanfic posting guidelines to address this topic. Currently it's in line behind forum upgrade stuff, moderator/disciplinary guidelines, and some minor rules amendments.
Until the site itself is reformatted/adjusted to accomidate the review system you go with how about a pinned thread? Set up free form "reviews, your preferences, beta v.1.?" sorta of thing with each author/artist has one post to write what they want with each story ect and edit that post with a time stamp to show when someone adds something new ect? You can see how we set up out areas when given no leash and draw ideas from that?
I think it would probably be easiest to have people specify within their own story threads, rather than having a central hub; makes it more convenient for reviewers, and easier for authors who may have multiple fics they'd like treated differently... that way they can simply add or change a request for any particular story while they're working on that thread anyway, rather than having to maintain a masterpost of sorts.
We'll definitely update and revise any "standardized" options based on what we see people actually requesting on their stories, though!