There is a way in LR although it is not without its drawbacks; maybe Qimage is the better option but FWIW the following will print faint cut lines exactly as shown in the attached:
- Page Setup: Set overall paper size
Print Module > Layout Style > Single Image / Contact Sheet.
Image Settings > Zoom to fill
Layout> Set Cell size to your actual image size e.g. Height= 6", Width =4"
Back to Layout Style > Select Custom Package
Cells > Add to Package > New Page
Adjust Selected Cell to 8"x6"
Drag page 8"x6" cell up into the Image cell
There does not seem to be a function to centre the new cell other than by eye and snap to grid; maybe I have missed something?
EDIT:There is another should work to centre your image within your required borders. Just add temporary 2" cells around your print area. Then add another cell around the whole of the image and temp cells. Now delete the temp cells and you should have your image plus your borders with cut lines all around the final print size. Hope someone comes up with a better solution!