I have been curious about a few of these for a while myself. You might find the following email to be of great interest. A while ago I contacted former Nintendo Power Editor-in-Chief, Alan Averill, in regards to some of the protos you mentioned: Taz Express, 40 Winks, Airboardin USA and the numerous GBC games that they reviewed and were never released. Here's what he had to say:

So i don't know how much I'm going to be able to help you here, but I'll give it my best shot. When we reviewed games for NP, we'd get them in a pre-production form, which involved burning the code onto a dummy ROM and then playing that on a special N64 (or GBC, or GameCube, or whatever.) Once we were done reviewing the game, the ROM would go back into the pile and eventually be overwritten by something else. So 90% of the games we reviewed never existed in anything more than electronic form.

Occasionally we'd get a hard copy of a game from a publisher, but those usually had to be sent back after a set amount of time. If it was a small publisher (or a particularly sloppy one) they would sometimes forget and we'd stumble across the title in the bottom of a desk drawer years later, but that didn't happen too often. We also rarely got manuals, and never boxes.

There could be review copies of the games you mentioned floating around somewhere, but they won't be something you can track down with any sort of normal search. It was a little unusual to review a game that didn't come out, but not completely unheard of. Companies would sometimes change their mind at the last minute (or go out of business altogether), and by the time we heard about it, the review was already out the door. I imagine that's what occurred with the games you're speaking of, although honestly, i can't remember specifically what happened with them.

Hope that helps, or at least saves you from trying to track down review copies that likely don't exist. Also, good luck with the collecting. I've got a little collection myself, and i'm always happy to help a fellow game nerd.