I haven't seen this speculated anywhere before, but my search wasn't exhaustive, so forgive me if I'm not the first to make this call.
There are too many suspicious circumstances regarding "Jack" as he is currently accompanying Parson in Jetstone's courtroom. Jack appeared back in Jetstone after Parson entered the portal, even though he had already told Parson he was out of juice in the MK. He never explained how he got past all those casters against their will without any juice; he just said he used "some sort of trickery." Conveniently, since Parson thinks Jack is out of juice, he won't expect Jack to perform any foolamancy.
That's convenient, because I don't think "Jack" is a foolamancer at all--I think he's Bowie, Unaroyal's changemancer who Queen Bea sent away. We've seen 2 of Queen Bea's casters already: Vanna the turnamancer and Jeftichew the carnymancer. Those happen to be the first two in Queen Bea's list to Don King, and the next one up is Bowie. We don't know much about changemancy at this point, but I bet it wouldn't take much for the caster to change his appearance and voice to match those of a different unit, i.e. Jack Snipe. I wouldn't be surprised if Spenser the Findamancer also made an appearance before Book 2 is over.
I know there are a few strikes against this theory, namely how "Jack" seems to know things that only the real Jack would know (Antium's name and status, the fact that Jack trained under "a certain smashing warlady," etc.), but I bet those could be explained away easily enough. I mostly just have a strong hunch about this and wanted to get it in writing before anything else was revealed, so I'm making this post. In the most recent comic (LiaB 203), Jack made a point of saying "I'm with you" to Parson. I bet some treachery is about to go down in Parson's own stack. We'll see.