...however if it is not defined, then the 'instrument adding screen' uses the OCP texture.

A seemingly easy fix would be to prevent panelTexture from overriding the custom panel definitions.

I have a separate texture for panelTexture and the custom panel, that way when instruments are being added, the buttons are there.

Proper panel vs. broken panel:

So panelTexture is supposed to override the normal texture, that's to allow you to use general panel layouts and still change the texture.

Now, if the instrument adding GUI isn't using a custom panel texture, THAT is an issue. Is that what's happening?

I see that is what's happening. Will fix.