someone was suggesting it might be Ferro, which would tie into the Legion Ring
Hmm. Well, the tapping he was doing with his bare hand did sound kind of metallic against that transparent barrier...
But what use would Zoom have for Ferro? His superpower doesn't seem exploitable in any useful way; large chunks of iron are easier to come by. Taking a Legionnaire's ring would enable Zoom to fly (which he hasn't been seen to do) and protect him from hostile environments (potentially useful) but stealing a ring doesn't require keeping its owner prisoner.
Maybe trying to question him about 30th Century Earth-2, which would be centuries ahead of Zoom's time, in hopes of exploiting future tech?