I have an idea for a plugin that would insert some code between HTML tags in the header section. Admins would also be able to modify what shows and what is hidden in those inserted code from the ACP. How would I go about this? This would be my first mybb plugin so please be gentle
.
see
this guidance - there are some other guides but I believe suggested guidance should serve your requirement
A simple structure of plugin (not a complete plugin) that inserts a variable with evaluation of a global template can do it - exactly what you want : matter of a minute to code.
I don't think such a plugin would be useful anyways, because header is the most modified template in all themes. Just look Menu Manager's reviews:
http://mods.mybb.com/reviews/menu-manager So it's always at least 10x better to edit template for such basic things directly - not only because of theme differences, but also it eats resources redundantly. For hiding just use HTML comments.
Its not going to be modifying the header templates. I want to insert it directly in between html tags, such as <head>.