In the paper Elements of Dance from Pre-17th Century Mediterranean and Near East Regions, one source quoted discusses the Gaditanae – the dancing girls of Cadiz – and their use of zil-like instruments, or bronze castanets. Without claiming that these instruments are exactly the same as zils played by modern belly dancers, we know that castanets are shell- or cup-like instruments with a pair held in the hand and hit together to make rhythmic sound, usually made of wood, but if they were made of bronze, as alluded to by Martial, they would be quite similar to zils. Ancient Greeks also had two similar instruments. Kymbala were small cymbals, claimed by  to be between 5 to 18 cm diameter, and it seems they were held in the hand and hit together. One source, Kotsanas Museum of Ancient Greek Technology, claims “the smallest had a leather loop in their centre for a secure hold of the player’s thumb and middle finger. The larger ones had the bronze rings in the centre (through which the fingers could be inserted) connected with leather straps or chain.” There are existent cymbal pairs small enough to be played like zils. This pair of Peloponnesian Greek 5-4th century bronze kymbala from the Metropolitan Museum of Art at 3 7/8″ diameter are roughly the same size and shape as the brass Saroyan Al-Taj at 3 3/8″. Like modern zils, the sizes of kymbala seem quite variable, such as 3 1/2″, 3 11/16″, 4″, 4 3/4″.
While it seems uncontroversial that cymbals always appear in representations as accompanying dancing, they are shown one in each hand, not a pair in the hand like zils. At best, we can say that a modern belly dancer with good zil chops could likely have played these cymbals as she plays her zils, but we cannot say for certain that Ancient Greek dancers did. However, the cousin of the kymbala and another Greek idiophone, the krotala, was played in the hand, presumably much like castanets, and is depicted played by dancers. See, for example, this red-figure kylix featuring a dancing woman with krotala from the British Museum.
The primary defining characteristic of the krotala seems to be the striking, staccato sound it makes, but the material isn’t always clear.  suggests distinctions between krotala, kymbala, and krembala may have been “flexible” and says the krotala had a wooden or “wooden-metallic” timbre. If metal and played in the hand, it might have sounded something like this. Taking into account the supposed differences in hold of kymbala and krotala, we can hypothesize that modern zil playing probably covers the ornamentation of both instruments; when played quickly and rhythmically, zils probably are producing the kind of accompaniment accomplished by krotala players, yet when struck and allowed to ring at key points in the music, zils are probably mimicking kymbala. But that is not to say that kymbala could not have been played more rhythmically; it is simply saying that if you have the ability to make sharp clacking sounds and ringing sounds (and we can do both with zils, I might add) you might use them in this way, but even these distinctions can be blurred. I suggest listening to “Song of Savarios” on The Hellenic Art of Music: Music of Greek Antiquity by Greek musicologist Petros Tabouris and listen to the metallic clacking and occasional rings. These could easily be made by two different instruments, or by a skilled player with cymbals played in the hand.
What we can conclude, then, is that these three hand-held idiophones – zils, krotala, and kymbala – share similarities, and our use of zils while dancing movements with descriptions documentable to Ancient Greece may not be as far-fetched as previously thought. At worst, it is a step away from “period practice.”
 T. J. Mathiesen, Apollo’s Lyre: Greek Music and Music Theory in Antiquity and the Middle Ages, University of Nebraska Press, 1999.
Update: shortly after posting, I found a few more references I wanted to include, so rather than creating a new post entirely, I updated this one.