I’m afraid there isn’t an interface for dropping extra bits into the GHPSEO configuration pages, but you’ve hit on something of a pet peeve of mine (soap box time)…
As you’re no doubt aware yourself, WP currently makes it very inelegant to add plugin configuration pages at all — that’s one reason why you see so much mixed XHTML and PHP in plugin code. A cleaner way of doing this would be for plugins to provide only data structures and textual descriptions for the user, letting WP do the job of marking up everything in the admin interface. (Yes, I know that’s what the WP developers say they’re already doing…but one look at the mixed code required actually to achieve anything, and you know better…)
In my own plugins (and in a few others out there), I’ve tried to bridge this chasm of looniness with a method enabling complete separation of data from markup, and I’ve personally found it speeds up development tremendously. However, extending this interface to open it up for other plugins to add to means going back to face that chasm of looniness again. I’m loathe to do that, as I keep expecting that any day now the WordPress core will be updated to enable plugin developers to interact with the admin side of things more elegantly.
Of course, I thought something similar when I first encountered version 1.2 and saw the mixed code in theme files, and there’s been very little advancement there either… There again, I develop my own themes with some bridging code that gets rid of some of that looniness, and at least one theme framework has sprung up with similar ideas in mind. But any day now, WP itself is bound to modernize in this area…isn’t it…?
All the best,
Greg