In fact, it's on my "Wish list". BATFE "legal" or otherwise...
With the US Vs Rock Island Armory being a "win" for firearms manufactures, I can't see the BATFE's arbitrary asshattery standing up to a decent court challenge.
The BATF has, on occasion, declared that a particular firearm fell under NFA'34 after having earlier found that it did not. Most such cases involved twelve-gauge repeating shotguns. When a firearm has been reclassified, the people owning such firearms have been required to register them, but were allowed to do so tax-free; any future transfers would require a $200 tax payment and other paperwork. Had the BATF been able to implement the same policy with the Accelerator, they probably would have done so.
The problem is that the way the FOPA is written effectively prohibits the BATF from grandfathering existing units while banning new ones. The inability to grandfather could pose an interesting problem, since declaring people's property to be contraband by fiat--especially after having previously declared that it was legal--constitutes deprivation of property without due process.