Seggos, the president and artistic director of Valhalla Wagnerfest as well as its founder, will sing the role of Brunnhilde, the once-immortal heroine whose awakening to Siegfried's kiss portends the dawn of a new age. The refurbished Capitol, its lobby resplendent, is expected to be air-conditioned in time for the event. Richard Woitach, a conductor at the Met since 1974 and an experienced Wagner interpreter, will lead the 75-member orchestra.

The $750,000 production features the tenor William Johns as Siegfried, a role the singer performed in the Met's production of the Ring this pastspring. Since the Capitol Theater is known better for rock and reggae than opera, and the shadow of the Metropolitan Opera looms large, several factors should be immediately noted. 5 and 12, the company will present an all-new fully staged production of Wagner's ''Siegfried'' - the third opera in the ''Ring'' cycle - with English supertitles.

Seggos's nonprofit organization, Valhalla Wagnerfest, found a home for its premiere season at the restored Capitol Theater in Port Chester. Negotiations with the county to present opera at the dam were unsuccessful, however, and Mrs. Seggos because of its ''imposing'' outdoor setting - well-matched, she felt, to the German composer's monumental works. Valhalla, of course, is the name of the mythical hall in Wagner's four-opera cycle ''Der Ring des Nibelungen,'' but the plaza also appeals to Mrs. WHEN the Greenwich soprano Lili Seggos conceived her dream of a Richard Wagner summer festival, the Kensico Dam Plaza in Valhalla was its setting.

Valhalla rock opera