Gly does its work within 1-2 hours of contact and it absorbs chemical. I spread seed then spray as I don't want seed sticking to any wet plants and not making it to ground. You can also mow couple hours after spraying gly with no prob with kill. Cleth and others require more time for absorbtion.
You can plant seed, by broadcasting or incorporation, and spray gly a (amended to read) a day or two later. It can land directly on the seed, or on the soil covering the seed, and it will not hurt germination in any way. To do anything, gly must contact tender, actively growing green stuff, or otherwise be applied in such a way as to be translocated to the roots of a plant/tree (google hack n' squirt).