I don't know how effective a pre-emergent would work on tree seeds (or how much you'd have to use). I have the same issue with red maples and seedlings come up everywhere. Sweet gum too! Next year you could try pre-emergent in one area and see if it is better. As long as you have things in there that you want to come up from seed!

Keep turning that mulch and pulling those seedlings. And consider getting rid of your locust. I'm in mid-whirlygig season right now..and every day I think of getting rid of the big maple in my back yard. Grrrrrrrrrrr.