You may be right. 6-8" may be too deep.
I'm trying to grow beans/peas in summer, but my fall plantings will grow in this throw-and-mow planting technique (buckwheat, winter peas, wheat, crimson clover) although germination rate is poor for the winter peas. Perhaps using the cultipacker will help with that.
We shall see what comes of the beans, buckwheat, Lab Lab and millet I broadcast and mowed growth onto the other day. Experiment, experiment and experiment some more...
Other than the crazy cost of a drill, I think the drill would have a real problem with the rock in my soil. Below is a picture of one of my plots with the least rock. Still very rocky. Most is fist-sized, but some softball, cantaloupe and even watermelon-sized rocks too.