I have used paint over inkjet decal paper and it worked okay. No seal coat needed for enamels- they are waterproof. I think most acrylics are too, as long as they have dried thoroughly. I have not tried decals done with acrylic paint, however.
The seal coat is to make the decal waterproof when you dip it in water, else the decal design will float away or smudge very badly. So as long as material is waterproof you are okay.
One thing to watch, however, is to make sure no paint applied to an edge. That is, you need to have some unblemished decal material to cut through when you trim decal. If paint is on edge of actual decal you dip in water, it may not release at that point and tear decal. Water needs to get through backing paper and release decal.