So, you might be able to take some scrap material and make some sort of object that has a fixed length, say 500mm, that hooks on the material and then the parallel guides hook to it, then you still use the guides as they are, but add 500mm to all the values.
I think you would have to cut something that is S shaped, then mount that to a flat board that can rest on your material to support it all. The Parallel guides blocks would now sit farther out on the rail to account for the S shaped object between them and the board. The S shape hooks around from the end, to the back of the sheet, runs along the side of the material, and then has the other end of the S sticking in front of the moving stops, this now mimics the back of a sheet of material, but with that fixed offset.