Well it's not peaceful but I can think of a way to pull off a union without a Byzantine Sassanid war. Have some of the royal houses of Iran flee to Constantinople from the Muslims, and over a few generations or so they christianise and then in about 800, lead the reconquest so that they all affirm the supremacy of the Byzantine emperor, after the Zoroastrian population is no longer hegemonic but before there's a Muslim majority either. I don't think it'd last very long but maybe it could briefly work?