This idea has been brought up and discussed on a couple of occasions, and there are a few reasons why this isn't something anyone has done.
First of all, one of the beautiful parts of portablizing is the uniqueness of everything. While Gman can crank out 4 of the same portable over the course of a couple of weeks, there are virtually no identical portables between people. The only one I can think of is Gman's and Shank's Pwii pockets. It's really awesome to see people design their own cases and esign things on their own, but this element would be completely lost with portable kits.
Secondly, a universal portable kits wouldn't be all that great because everyone wants their own different tweaks. GameCube buttons? DS buttons? Wii U gamepad buttons? 5 inch screen? 4 inch scree? 7 inch screen? 3DS sliders? Real joysticks? Everyone is going to want everything g just a little bit differently, and designing 50 different cases for a person to choose from is pretty impractical. Even if you restricted it down to say three different options, you'd still probably end up perfectly accommodating no one.
Additionally, no one in the community is really in a great spot to do this. You'd need to build up a decent amount of capital and interest from other people in order to make this a reality, and then you'd have the same issue with people wanting different things.
And lastly, I'm not sure how much this would actually help new members of the community. They'd have to make this big, single purchase, which generally people don't want to do, and then there's still a decent amount of skill that goes into just assembling a portable. Odds are that some people would kill a motherboard, or melt a part of their case with their soldering iron, or be using a $10 RadioShack iron and be mad that nothing goes together like it should. This would mean that the person running the rodeo would have to take orders for replacement parts, get money from whoever is having problems, and likely have to answer a lot of questions. And also, learning skills like case design and fine soldering is really great! When I first started, I really, really wished there was a portablizing kit like what you're suggesting. But now, I'm very glad that there wasn't a ready made kit for me to buy when I started, because I've gotten to learn all of these things, make mistakes, and grow all on my own. If I had been handed a ready-to-go kit, I would have broken so many things, and wasted so much money. By doing it piece by piece, I saved myself a lot of frustration.
Now this isn't to say that such an event is impossible.
Downing has hinted at what I believe is a trial run of something like you're suggesting, where he gave N64P kits to some people and they are building their portables off of what he has given them. I just don't feel like anyone here has the resources or interest in undertaking such a large scale project in making portable kits, but I do understand where you're coming from.