I like the idea, but I believe there are indeed already journals on the FAA.
But you also need to understand that anything you post in this journal is not private, and others can read it. They could restrict other people from posting in it, but there's nothing they could do to keep others from reading it, unless they made a section for each individual person, which I'm highly doubting.
Also, you can make a journal in your den, if you have one. I think the better idea is to have a public den simular to TLKP's, to where people can post anything they can post in a den. Also, I know you can add blogs to phpBB forums, but I'm not 100% sure what version you have to be at to impliment this feature, considering when phpBB adds things to their updates, some older things become unworkable.