The Multiply blend mode is a good technique to combine line art with textured, alternate background. For example, if you have an original pen and ink drawing, you can scan it in, place the layer on top of a new background or on top of different colors and then set it’s blend mode (on the layers palette) to Multiply to combine the images. I’ve even seen people apply “temporary” tattoos on photographs using this technique.
Here are a few examples of when I have found the Multiply Blend mode useful in my own work. Although none of the blended layers are "line art", they are all high contrast photographs with white backgrounds (or almost white) which blend easily. The first is a photograph taken in the Forbidden City in Beijing China composited with a motion blur image of the ocean, the second were bats flying in Austin, Texas right before sundown (composited over a second photo of the overpass), the third, a photograph of a target at a shooting range, multiplied with a paper texture with the Chicago skyline painted below, and the fourth a silhouette of trees in the fog in central California multiplied against a prison wall in New Mexico being lit by window light.