Hey guys I'm new here, but I've been lurking for quite some time.
Now all of us know about the friendzone, and back in our AFC days I'll bet most of us ended up there to our own dismay. But we also know girls that we've put in the friendzone, and I can only imagine they experience the same dismay.
Obviously girls know what the friendzone is, and they've most likely related the word itself to the negative feelings that could be deeply associated with it. Girls think they control who's a friend and who is a fuck, and therefore when they're verbally told that they are in someone's friendzone, (in the right time of the pickup of course) then they will experience confusion, despair, (if they're attracted to you) and/or hopefully, panic.
I have field tested this on only 2 girls so far, and the look in their eyes as well as their micro-expressions (not enough posts or I'd give a link, just google it) showed the three feelings described above. As a manager, I was taught the "knock 'em down then build them up" technique to increase motivation and loyalty from my subordinates: verbally scolding/abusing them enough to the point that they feel ashamed of themselves, then bringing them back up by telling them their strong points and praising their abilities. This is great because it carries them through two completely different emotions.

I think a great way to use this is when a girl is giving you kino/returning kino. Personally I think it would work as well as/better than "it's way too early in our relationship for you to be doing _______" but that is the scenario in which this technique should be used. HOWEVER, after you knock them down you can easily build them back up with the possibility of them leaving said friendzone. For example:

HB: (touches you/returns kino/gives flirty smile etc.)
Neb: Whoah easy now, don't touch me like that/look at me like that, you're in my friendzone.
HB: (gives perplexed look)
Neb: Hey don't give up so easily, there's still a chance to get out of it... if you're good :P (Best way to deliver this is cocky funny or with Gamblers tongue in cheek method)
HB: (smiles/grins etc)

She's probably never been told that in this situation, and that's how she goes through the different emotions. You can also leave out the part about her getting away from the friendzone out of the conversation, which will most likely cause her to attempt DHV to you.
Now if she says something like "Oh i'm in you're friendzone??" don't necessarily judge the words she says as much as how she delivers them. If she's tongue in cheek/flirty and maintaining eye contact then you've probably attracted her. But if she says it in an unfavorable tone and rolls her eyes or looks away, then you probably didn't build enough attraction.
There it is guys let me know if I didn't go into enough depth or vice versa, if it's been used before, or just tell me it's dumb haha.

Overall I like it. It's definitely a disqualifier that has been advocated by the likes of Style and Mystery.

The only thing I'd tweak is your conditional statement ("if you're good"). I'd stay on the disqualifying path a tad longer. Fortify your first disqualifier (she's been friendzoned) with a reason why (doubling as a second disqualifier): She's too much of a good girl for you. It's a flipping of the script alluding to the fact that nice guys get friendzoned by attractive girls. Then I would qualify her with something from the "what's the craziest thing you've ever done" genre of questions.

I agree, but I would tweak your tweaking by changing it to "I only get romantic with ___ girls." (spontaneous, adventurous, something that doesn't concretely disqualify her)

She'll make an effort to prove herself, rather than trying to change something she may like about herself (being a good girl or whatever)