Is it possible to create/correct a simple trapezoidal distortion?

[denzjos]

@Esko, have you tried the Ottia Tuota plugin :http://kmarkku.arkku.net/Perspective_transform_files/Gimp_master.html
Because the pictures you scanned have all the same trapezoidal distortion, I suggest to try the '4 points to 4 points perspective transform'. Make a rectangle path and make a second path following the distortion on the scanned image and then use the plugin. Measure the result (transformed image) with the gimp ruler and, if it is necessary, correct the image with the scale tools to have the correct dimensions and write down the scale values to correct the following images the same way. Use the two paths you made for every image you make on the same scanner.
You also try the Ottia Tuota 'perspective transform - 4 points to rectangle corners' plugin. You only need one path with this.
It's just a suggestion.

[Ofnuts]

Yes, as others have mentioned, I have encountered a similar problem with my V200 scanner (it seems common to many scanners, and in particular those with good color rendering) and made a script/procedure to fix it (because it's a bit more complex than a trapeze transform). See here.