RobA
That MM script do exactly the trick i just try now
BEFORE
Attachment:
Untitled-3.jpg
AFTER
Attachment:
Untitled-3a.jpg
the black lines were already on the original so is perfect.
I didn't play much with the MM composer but i feel very tempted now with such good ingredient
would be possible edit the script to add a option to increase the layer size instead then mantain the original size (yes i know will be much bigger and could be very risky to use such options on already large image

... )
Rob's result looks identical to using Simple's Center Tile plugin. Cool stuff.