It's a nice idea micker. It would be great to be able to have more image effects available within Scribus without having to go outside the application.
However, since Scribus automatically picks up changes to images and updates them - almost - on-the-fly, making the changes in GIMP and re-exporting - a simple keyboard shortcut - has pretty much the same effect.
I know it's not quite the same thing but it's "free functionality" that doesn't need to be kept up-to-date with another bit of software.
Having said all of that though, I've no idea how easy or difficult it would be to actually make a Scribus G'MIC plug-in happen. It could be simple, or it could be a nightmare. If enough people say that it's something they want then one of the developers might look into seeing if it could be done, you never know...