As long as you can dig freely in the soil without encountering a lot of roots, Japanese maples should grow fine under pines. The filtered sun that you often find under tall pines is probably close to ideal light conditions for many Japanese maples. Some of the red/purple varieties might need more sun to achieve their best color, but most prefer at least some shade.

my dissectum is doing fine under my pine tree
like dawgie said, if you can dig freely, then it should do fine
but if your spade won't go more than a few inches deep with each try, don't even bother

I think the key is the amount of moisture that it receives--remember that Japanese Maples like moist but good-draining soil (they are native for the quick-draining, monsoon-drenched storms of Asia), so U don't want them to be too dry. Also remember, though they like frequent water, standing water is a threat to virtually all maples (they can "drown" quite easily). Ditto the prior posts on the root situation.