There are loads of ways you could do this. I'd probably export the grid as a .tif file of the same dimensions and then use ImageMagick to combine the images (it can read SVGs directly, but it's not so good at rendering them). Here's an example:
graph.tif:
grid.tif:
command line syntax:
composite -compose Multiply graph.tif grid.tif graph2.tif
Result (grid2.tif):
You can use command line scripting to apply the same grid to multiple images, e.g.:
for (( i=1; i<=4; i++ )) do
composite -compose Multiply graph-$i.tif grid.tif graph2-$i.tif
done
This will add grids to images called graph-1.tif, graph-2.tif. graph-3.tif etc. in the current directory, and save the results as graph2-1.tif, graph2-2.tif. graph2-3.tif etc.