I also tried and got the same response.

Poor response from wizkids but I do think the comment on multi target mind control supports the no side of the argument so that's the way I'd rule it in the future.
I hope questions like this and the recent thread about powers that trigger at the beginning of turn suggest to wizkids the need to provide clearer terminogy for the order and resolution of "events" within the phases of the game in the next rule book, it seems to me a lot of these types of issues could possibly be dealt with by tidier game language.

Which comment was that?

If anything, targeting multiple characters with Mind Control would be pretty analogous to this situation and in that case you can most definitely have one MC'd character carry another.

The simplest way to do this is for Black Mask to use VAST FORTUNE to give Bane and Firefly a free move action. With that free move action Firefly carries Black Mask and Bane moves adjacent to Firefly, then as a normal move action Firefly could then carry Bane into a new position.

I was going with this comment from Lantern jordan as it suggested to me you need to that was the way he was thinking but maybe I misinterpreted his intent, its possible I should never get into rule discussions after a 12 hour night shift.
That said seeing as Wizkids have washed their hands of the issue to quote He-Man " I have the power " I'll play it the way I think is right and stop wondering about it till they inevitably do the same thing again with a modern age figure.

definitely still in night shift addled brain mode had to read this a few times for it to sink in then I had an epiphany (well it is Christmas)I may be wrong but I got to a sort of yes we are all right for all three opinions in this discussion.

if you have the opinion that both figures get a free move because they both started off adjacent and it doesn't matter where Bane is when he uses it then it works

if you have the opinion that Bane needs to be adjacent when he starts his free move action then it just becomes a timing issue on the free moves if first Bane moves then Firefly moves adjacent (carrying Black mask or not) then that works as well

if you have the Wizkids opinion, what are you doing playing with those boring old figures do what you want and stop bothering us unless its about the shiny new figures we want you to buy, then you do what you like and it still works

so everyone is happy and there's no need for disagreement here on the realms, it's a Christmas miracle.

Id say you have to move them both separately as free IMMEDIATE actions. THEN Firefly could carry Bane.

Bane isnt adjacent to Black Mask after being carried away and that to me makes him not qualify for the power anymore because you took an action that does, then you're trying to doing another one that doesnt since there is no adjacency (regardless if you were when it started) so the IMMEDIATE portion is kinda being ignored.

I see it similar to Spidey or Batman Enemy. You can start next to a character to get the perk but if you running shot away you not adjacent anymore and it doesnt carry over even if the power action began adjacent to the character you were borrowing the attack stat from.

Basically, after Black Mask uses the Vast Fortune power, is Firefly adjacent to IMMEDIATELY use the power? Check. After the free move action IMMEDIATELY resolves in which Bane is carried...anyone else adjacent to Black Mask to immediately use the power? No? Vast Fortune power and the free movements it gives resolves and ends.

I'd think similar logic could carry over for this ruling...at least that's how I'd bandaid rule it in the mean time.

I'm sticking with my answer, but revising my reasoning to something I think sounds a little more 'WizKidsy'.
The check to determine which characters qualify for the effect is performed at the time the power action is given. Each qualifying character will be able to be given the free move action regardless of a later change in their qualification.