The D200 has a rectangular eyepiece frame. The D2x has the round Nikon threaded eyepiece. My guess is the rectangular frame is a standard JCIS design with side grooves for accessories.
The Nikon site (Japanese translation) describes the DK-21m:
The eyepiece which when approximately 1.10 times (time * XXINF * -1.0m of 50mmF1.4 lens use -1 being) expands finder magnification ratio by installing in the finder ocular section of the single-lens reflex camera itself.
At only 1.1x magnification, this sounds like single element optic housed in a clip-on eyecup. I couldn't find an image. The D200 eyecup is like the D70s eyecup on steriods. It is rubber bonded to a clip-on retainer.
Where'd that come from? Loctite makes many products, some of which are nearly super glue, some of which are fairly weak. I don't recall the color coding, but the weakest loctite would probably serve admirably to help the part stay on and yet be easily removable. How this should threaten one's eyesight is beyond me, unless you think Len just advised someone to tip their head back and drop Loctite in their eyes like fake tears. Likewise I don't see how this will void a Nikon warranty. A little weak Loctite used sparingly and allowed to dry before using the camera should present no risk to anyone or anything.
As with anything, it might be advisable to put the tiniest of drops onto the plastic you intend to use it on first and see if it damages it through solvent action.
Just FYI, It's not anything remotely like super glue. Some may know it as thred locker. It's commonly used on eyeglasses! Anything where there is constant torsional force on a screw or bolt. The said screw can even be removed after the fact.
Well, just like with any type of glue one must be careful not to glue one's self, unless that is your intention (I use Dermabond, which is cyanoacrylate, aka Super glue, to bond skin lacerations.)
But clearly, there was nothing 'dangerous' about Len's suggestion. Most of us have sense enough not to glue ourselves to our cameras.
For the record, the preferred Loctite for this would be the 'blue' variety. It does not harden, per se, and maintains a rubbery consistency. Even if you were enough of a Homer (Simpson) to put your face to the VF with blue Loctite all over it, you'd only have a bit of a mess, not a health hazard.