There’s no easy way to do this now. You need to tap into the formCategoryDropdown() function but we don’t have a filter for it so you would need to modify the core to change the category selection at all. I’ve just created a ticket to add this functionality in a future version and we’ll likely add this functionality soon as it’s a pretty important function to be able to control what categories are displayed.
I think I will just leave that change to the developers. It is most important for my site that all the categories be shown. I know that they are all there, but I don’t know that my users will. It would be easier for my users to see all of their options to begin with, and not to have to click something to see more.
Thank YOU for all the feedback you’ve provided on the forums here, John. You’re helping us make a better product with each bit 🙂 Let us know if we can do anything else.