I see them nonchalantly displayed and sold at gun shows in all the convenient calibers. I suspect that the ATF can and, most likely, will issue a ruling addressing solvent cans at some point.
When I say "ruling", I mean closed door session where the decision becomes de facto law. As I understand it, the ATF (much like the IRS) has the extrajudicial power to rule on areas that fall under their umbrella of governance.
My hunch is that the ATF knows all about what's going on but have yet to prosecute for lack of resources or they're too busy do other things at the moment. But some bureaucrats love pet issues like this and all it will take it is one or two idealistic, anti gun paper pushers to go "full chub". At that point, I'd be too spooked to even have an innocent ol' oil filter around, let alone the threaded adapter lest I be guilty of "constructive intent" or whatever they're calling it.
As it stands now, you're probably in the clear to have one. Heck, even attach it to the muzzle and use it to catch solvents. But these idiots on youtube who actually pull the trigger without a stamp are going to be SWATted hard at some point.
Even if you they win the court case, how much time, energy, and money have they spent? Was that time in jail worth it, even if they are exonerated? I'll pay the $200 and get a legitimate suppressor, thanks.