indeed, I forgot to mention that the trouble is in agreeing with the meaning of "trivial".
But there's no need for project maintainers to value contributor's code. Let'em do it themselves. Offer two sets of terms under which you will accept contributions: copyright assignment (for "trivial" stuff) and copyright retention for "substantial" stuff), and let their common sense do the work. And if you feel like it, you can always retain the right to reject patches offered under the "wrong" terms.