Yes an 07/02 FFL/SOT can take your gun, register it with ATF on a Form 2 as an AOW, engrave his info on it (since he is the manufacturer of the AOW now), install the foregrip and then transfer the gun back to you on a Form 4 with a $5.00 transfer tax. However, he is not going to do all of this for free. Cost may be less than doing it yourself, but far more than just the $5.00 transfer tax.
No that is wrong.
A $5 transfer is to transfer an AOW (all other weapons) from one registered owner to another.
As a Manufacturer when I register a weapon it simply tells the feds that i made a weapon and what the serial number is. It does not make me the registered owner.
When I sell you an NFA item that I made then You are the first owner and must pay $200 to register it, even if it is an AOW.
but then you can sell it and transfer it, If it is an AOW the registration is only $5, but still $200 on all other classifications.
There is a lot of misinformation about this out there but if you want to hear if from the feds then read the National Firearms Act or at least Title 27 CFR part 279 of the Federal Firearms Regulations.
Donald