As long as you stuck with the "straight tube and weight" type of grip extensions, I think Freedom Arms would be able to get the ATF to sign off on it not changing the status of the revolver. (In other words, it wouldn't be a shoulder stock that made the revolver a short-barreled rifle.)
The problem is that FA isn't going to build anything for you until they have that determination in writing from the ATF. And, unless they see a decent number of export sales coming from it, they won't be very motivated to send the determination request in.
Of course, if they could produce them as an "export-only" model that's not for sale in the United States, it would simplify things. But, I don't know if such legal avenues exist.