You will never get a concrete answer. even with the people in the DOJ or ATF or wherever. ALl of the products produced today bring up boundries and push the limits of legal definitions/intensions/literature. You can propose this question to different people at different agencies and get a handful of different answers. You can throw it at different DAs and get different answers.
People even push the envelope when they mount BUIS on the underside of their rail for a hand stop, technically it can be gripped in the same manner that technique has shown above. NOw I know that is not what a BUIS was made for but it is the manner that it is being used and the fact is that it is being used as a FG.
it is just a matter of time before somebody is the sacrifcial lamb and gets slaughtered in court by some over zealous DA or ATF agent.
When in doubt... leave it out
I am in the same boat and would give anything to throw the AFG on my pistol but I fell that its not worth the risk