Lisa Kies of SIG was kind enough to respond to my query. Unfortunately, her most definite piece of information was that the brooches aren't associated with the Slavs or the Russians, either. She observed that the one I'd found was Mordovian. She told me, "According to George Vernadsky in Ancient Russia, the Mordva were an East Finnic tribe living in the Volga area."
Wikipedia notes that there is a Republic of Mordovia located in the eastern part of the East European Plain of the Russian Federation. Wikipedia also notes, about the Republic and its residents:
The Mordvin people are a Finnic group speaking two related languages, Moksha and Erzya. The two languages have been dealt with at various times as dialects of one Mordvinian language. In reality there are two orthographies with parallel newsmedia in the Republic of Mordovia where approximately only one third of all Mordvinians live. During the Soviet period, school textbooks were published in each language.
Earliest archaeological signs of human beings in the area of Mordovia are from the Neolithic era. Finno-Ugric Mordvins are mentioned in written sources in 6th century. Later, Mordvins were under the influence of both Volga Bulgaria and Kievan Rus. Mordvin princes sometimes raided Muroma and Volga Bulgaria, and often despoiled each other's holdings.
Wikipedia goes on to relate that the Mordvins were later subjugated by the Mongols, and eventually by the Russians, leading to their political position today as a "Russian" republic. As Lisa said, though, they are neither Slavic nor Russian.
I'm not sure how to go about verifying whether most of the Omega brooches have been found in traditionally Mordvin lands. The vendors of such antiquities tend to characterize them as "Viking" (the ones I've seen are either 9th or 11th century "Viking") without providing the slightest rationale for such a characterization. No, I take that back. I assume they call the brooches "Viking" to improve sales. But that sort of "information" does not help to ascertain who wore the brooches, and how. Many of the antiquity-sellers don't even bother to state where they have been found, or if they do say something vague like "Northern Europe," as is the case with the Omega brooches listed on this page.
This page, surprisingly, is somewhat helpful. It refers to the item as a "Baltic 'Omega' type" brooch, says that it was found in the Lake Ladoga region and cites the following book as a reference: Sedov, V.V. Finno-Ugri i Balti v Epokhy Srednevekoviya, Moscow, 1987, p.292 fig.12.
That seems to confirm Lisa's belief that the brooches are associated with the Baltic, though probably only with certain Baltic regions and tribes. I don't read Russian, but if I can lay my hands on the book I might be able to puzzle out the captions on the illustrations and some of the text with Google Translate. It's a start.