Can I attach a rear cargo rack directly to the seat clamp?
Before dual eyelets on the seatstays, using a single strut going to the seatstay bridge was common. Most racks either have the holes in the center for this or can be drilled. Then you flatten out a twisted strut with a vise or pliers and put a 90 degree bend in it at the end. Fine for all but heavy loads and much neater. And presuming this is some kind of racy 90s GT hardtail like it looks, it probably has short chainstays and you wouldn't really want to put a heavy load on it.
Doing what you propose could also work. It would need extra long struts on many bikes. If the struts are really long, the triangulation from having two of them might yield little or no stability advantage over having just one running to the bridge, which will be much closer. I would do that last before the above or just running p-clamps on the stays.
Using this area as the upper strut attacment point is good to do on really small bikes when it's the shortest run for the strut(s). That's when anything else would be a contrivance. So that's the good application of the rack-mount collars or of doing what you propose.