Let's just start this off straight away and say that the Cross was not debarred from use during reigns of either Leo III or Constantine V. Constantine V's coins replaced the emperor's portrait with the Cross on the steps. Today this is believed to represent Heraclius' reinstallation of the True Cross in Jerusalem after the defeat of the Sassanids. It could also be Jacob's ladder, probably the former though.

Secondly, we are actually hugely unsure to the extent that iconoclasm was actually enforced. There is a huge difference between destroying icons and simply not believing in them and not creating new depictions.

The most famous example is the Hagia Sophia church in Nicaea, modern Iznik. The church no longer stands but it was photographed in the early twentieth century. An earthquake in Constantine's reign brought down the apse with its depiction of the Virgin. Constantine put up a black Cross mosaic on gold tessarae.

So much of our knowledge of iconoclasm comes from the iconophiles that what they actually believed is hard to know. Inventing Byzantine Iconoclasm by Leslie Brubaker is the book that states that it is mostly a fabrication by those who created later condemnations. (After the chronicon paschale and Sebeos finish in the late seventh century, we basically have Theophanes annals and a much later short history of patriarch nikephoros as the sole Byz. authors until the ninth century).

Without going too much into the theology, Iconoclasm (as it was portrayed by its enemies) was a rejection of human depiction as akin to idolatry. The Commandments of Moses of course do forbid bronze cows. On the other hand, unregulated cults and icons had propagated massively between the fourth century and the eighth. Early Islam also did not prohibit depictions, though it swiftly would in a few generations. If Islam banned images first, or if Byzantium did is actually slightly unclear but the Byzantines had to find an answer as to how they had lost three quarters of their empire to the followers of a different prophet and God.

So to answer, absolutely did they use the Cross. On coinage, the most common imperial means of communication, and in churches. Hagia Irene in Constantinople today still preserves it's bold stark cross created during Constantine V's reign. I hope this helps.