Idea is cool. But I'm not sure if allowing to create community posts directly in App Inventor builder is good for User Experience. Because you might need to create a full rich-editor like we have in Discourse. (Quoting another post, selecting category, inserting time, local and cloud drafts, using markdown etc.) So when users visit the community, they can see their posts looks exactly like what it looks on "Create community post" dialog on builder. Also, after creating the post, how users will read other replies? Will they need to visit the community?
So, instead of adding this feature to builder, I think this can be added to community directly with a Discourse plugin. (It can be also make things easier, maybe?) So you won't need to deal with creating a new account because both Discourse and App Inventor already manages your login credentials, you may just need to add a new endpoint on App Inventor side that returns projects with community login. (Of course, users will need to link their community accounts with builder accounts, as their email addresses may not be the same on both.)
The plugin can add a new option to the editor toolbar for picking a project to send as attachment (and some categories can have enforcement like at least one project should be selected, etc.)
Example: