In this case I think "easily reversible" and "no" would be synonymous in NY. And there is no "NYS approved" method. So you're, or anyone elses idea of what will "fix" the stockm in place and what a DA's idea in a hot media anti-gun climate may be two different things. The only way NYers get jammed up with things like this is get pinched for something else(domestic etc.) Guns get pulled and it gets sent off to a forensic examiner who will make a determination if it's compliant or not. If you were going to fight it(the AWB charge if they deem it an assault weapon) you'll need a damn good pro 2A attorney and an expert witness for your side. Spendy, but if you've got money to burn....
Which begs a few questions...
Why not avoid the potential hassle? Save the extra coin and buy a preban. The amount of $$ you're saving by doing the build yourself will most likely be the hourly or bi-hourly rate of the lawyer you'll hire to defend you.
Are you married to the idea of a non-folding folding stock? The underfolders provide a lame cheek weld. Put a fixed stock on it and enjoy shooting without a stock rod pressed into your cheek bone. It gets lame quick.
If you're just going for the underfolder look I completely get it. But the potential for entaglement, while minimal, is just nice to remove completely.