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A few comments...
First of all, LEGALLY there is no such thing as a "80%" lower receiver. It's a term invented either by the manufacturers or by gun enthusiasts.
As far as BATFE Technology Branch is concerned a so called "80%" lower is nothing more than something they've ruled to be NOT a firearm.
In summary, who is going to care if you add Cyrillic markings to a block of aluminum.
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This is the long and short of it.
However, there is also another part of it that is geared towards the difference between manufacturing and gunsmithing.
There is some confusion and of course some BATFE wording that doesn't specifically spell out that certain process are not manufacturing so some businesses are taking a conservative approach and saying they can't do that.
In your situation, the bad things that might happen if BATFE decides to bring the hammer down would all be on the part of whoever did the work.
If you search around there are definitely businesses out there that will do what you want.