Round-up is non-selective, killing both broad and narrow leafe plants, basically anything green with leaves.
I do think you could spray an area (say this summer) and then plant it later in the fall.
I would recommend that you do that early summer if you plan to plant in the fall.
I know for sure that Larry harding disk up big areas of woodland and sews seng seed and then mulches with straw.
In the spring the seng comes up all mixed in with lots of weeds including wheat from the wheat straw.
He sprays something that kills the narrow leaf stuff (selective herbicide), but does not bother the broad leaf stuff (including the seng).
I expect that if you spray roundup on seng, it will surly die.
I do spray it in a ditchline that I have at the edge of my yard and it kill everything there, broad and narrow leaf stuff, kills them all the same.
I had a spot that I picked out early last spring that I wanted to plant some seng in - it was ideal except for the forest floor being thick with poison ivy (hate that stuff and it bothers me bad), so early last summer I sprayed the heck out of it with roundup, waited 2 weeks and sprayed again, just to make sure I got it all.
I planted it last Octber/November and had excellent results there.
TNhunter