I needed to add a custom metabox to an options page myself, but wasn’t satisfied by using output buffer (seems like a hack!)
I dug through the option page source code and ended up with the following functions to:
1. Create the (sub-)options page
2. Register a metabox when on that page
3. Display content within that metabox
4. Save the values of that metabox
It’s all very straightforward in the end. As far as I can tell, there is no hook equivalent to “add_meta_boxes” for ACF. And that hook does not fire on the options page, yet you still use add_meta_box to hook them in.
Here is the code: https://gist.github.com/RadGH/a1473a24782e93435951ef0f390deb2e
