Guild of Writers

I know its currently not possible (as far as I know anyway) but would there be a way to in the future have it so that Journal (as in ones fan create) could be added to the second Relto Bookshelf?

Since that Shelf is not at all controlled by Python, not much will change there I think. It is however possible currently, just like you can add basically anything you want to any other age. Some (many?) explorers and writers, myself included, would not like such changes though as it would mean altering an age without even asking the author. In addition, people can completely destroy the "feeling" of the age and there could even be a fight for whom is allowed to do what, as the amount of space and ages to alter is a limited resource.
I am aware that Offline KI is shipping modifications for the hood and the city (which are however the only ages where explorer-made files add anything visible - all the modifications Dustin and Race did to make it possible to explore the Myst V ages do not add a single drawable object, and the remaining changes to existing ages are or will [with Drizzle 20 and Offline KI 2.8.2] be automatically converted by Drizzle, so they are actually Cyan creations). These changes however are just copied Cyan models and textures which got re-exported with almost no changes.