Thanks Jon, I'm fairly new to GIMP too, so we're pretty much in the same boat.
You can save the mask as an image by making sure, in the layers panel, that is the only layer with it's eye icon open, then just export that as a png image to preserve its transparency.
Again in the layers panel, just select the scorecard layer and hit delete to remove it.
To import another scorecard or the mask, click on 'file' (top left corner) and select 'open as layers' then search for your required image in the search dialogue window, once clicked on, your image will be imported into the layers panel. You might need to resize it though unless all your scorecards are the same.
Once you have the scorecard and mask looking the way you want, just export the whole thing as an image to whatever directory you want, and from there you can paste it into the OCR tool.
Hope that helps.