I’m not sure SCART can output 480p with the Dreamcast. Technically, Scart can go as high as 1080p (according to wikipedia) but I think YOU HAVE to use a VGA cable to output 480p when it comes to Dreamcast. Unless you have RGBHV through the SCART but I doubt it.
So maybe OSSC can’t do RGBVH through SCART.
So, I think OSSC doesn’t accept RGBVH through SCART. Wait for a more knowledgeable person to confirm, but it seems to me that the issue comes from here.
Normal SCART cables made for the Dreamcast are wired to make the console output in 15kHz (240p/288p/480i/576i) RGBS, and VGA boxes and cables are wired to make the console output in 31kHz (480p) RGBHV; so, normally, the Dreamcast can’t output 480p over SCART, because, by triggering 480p mode, you change the sync output from composite to separate, which is fundamentally incompatible with SCART.
That gets into the other part of your post, to which I can say no, the OSSC doesn’t accept RGBHV over SCART, because SCART was designed for composite sync (RGBS, YC, CVBS), so it doesn’t have the requisite wiring to accommodate a second sync line. Technically you could just send it over one of the otherwise unused wires, but no standards-abiding SCART device will be able to understand the signal.
Also, on this note, while SCART cables are indeed technically capable of transporting up to 1080p, as well as other signal types, like YPbPr, you need to make sure your destination devices support what you’re feeding them. The OSSC can absolutely handle 1080p over its SCART input, but that doesn’t mean all of your SCART-compliant devices will be able to understand or be compatible with 480p+.
As for the Toro, it’s not actually getting RGBS out of the console; what it’s doing is receiving RGBHV from the console and running the sync signals through a sync combiner to get RGBS, which it outputs on the SCART port. The OSSC can handle 480p on its SCART input, so this arrangement works (and arguably works better than using a VGA cable and AV3, because AV1 has better LPF support).