Hmm, yeah guest posting is the tricky part. You don't want people to be able to post things that are rude or illegal, probably not advertisements either. So it's generally a good idea to generate a hash cache type system to help prevent bots on submission spam.
To be honest, and I'm unsure how people would respond to this, I'd actually almost prefer to make the code a compiled php code module, rather than open source. Don't get me wrong, I love open source, but the issue is when any software becomes well used it tends to be targeted with automatic bot scripts designed specifically to get around any security measures that the software may have built in.
Generally now days, the easiest way to stop bots is to ask a question that's required for registration but requires at least a small bit of human intelligence. Sometimes even a simple math question suffices, but even that can be fooled. So I'm thinking that at the very least I'm going to allow the owners the ability to set a list of optional security questions and answers, one which would be randomly selected upon registration to help prevent bots from just making accounts to spam things.
But people would need to keep in mind, these would need to be question/answers that can be solved easily by a human being. For instance a question like "What do you get when you add the numbers one and two?", where an acceptable answer would be "three" or "3" (minus quotes of course). I'd make answers forced to lower case to keep from a case-sensitive issue too. There would also be no default questions included. The reason again is, its easier to write a bot for pre-written questions to break security.
That said, it may be noted, I'm a security nut. The reason being that my primary job is dealing with the checkout and order back end of an ecommerce software that had a little more than 4 million dollars run through it last year alone.

That said, I've also got experience with writing store stuff, so I may include that as an addon module at some point, but that's nowhere near even being in the initial design document (though allowing ordering prints would be neat).