I did some looking around but I am not seeing anything obvious.

If there isn't a thread out there already...Can you all share your clip-on knowledge with me?
What's required to make it work?
Which brands are worth considering and which ones to avoid?
What models are available that work with magnification? ...And what factors should I know about there?
Any models that are almost always not supposed to be in the public hands so don't buy because it's probably stolen .mil gear?

I guess the first question is what do you mean by magnification?
If you are talking longer ranges consider the ones listed below. Obviously the amount of star light and moon light play a role in all.  


OSTI PVS-27
L3 or Eotech CNVD-LR/M2124-LR
KAC-PVS-26 or PVS-30 refurbished or new. Watch out for PVS-30's in FDE at cheap prices. Check serial # with KAC. KAC units are heavy units but built solid. KAC has one of two more models for long range.
Eotech LR
OSTI PVS-22

PVS-27 can suffer from a lack of collimation. Jay at Sure Shot NV offers a service to correct for this. As I recall it is approx $1300-$1400. He can also retube the 27, 26 and 30 if you are looking for a higher end tube.  

You want your day scope/mount to be as close to centerline with the NV Clip-On as possible but some allow for more variance.
The quality of your day scope glass plays a role in using NV Clip-Ons

I have owned a KAC 26, 30, KAC UNS-SR, PVS-27, (2) Eotech PVS-24's and a PVS-24 that was retubed with a higher end WP tube by Jay at sure Shot NV.
All had there place with my favorite all around being the retubed PVS-24 by Jay as my hunting areas are limited in range.    


Search the net using sniper hide in the search for posts regarding NV Clip Ons as there is more info floating there.

If you have not seen these vids yet they are worth a watch. He is a member here and show what a skilled shooter can do with them.