Hi Banzai,
You asked this question: "I'm considering Koichi Yamaguchi's school, but, as a reasonably intelligent individual with a knack for fine detail and for making things, I wonder if I could teach myself just as effectively."
There certainly are a number of fine amateur and professional framebuilders who have taught themselves so it is indeed possible. But I believe they are the exception. When you added "just as effectively", there is no chance you can DIY as well. I am one of those that regularly teach framebuilding classes (I did my first one in 1976) so of course my opinion is biased but I think the most sensible way one can make their own frame is to take a framebuilding class. What intelligent person would argue that experienced trained teachers are worthless? That trial and error is as fast and effective as learning from an expert?
Building a bicycle frame is a bit more complicated than what beginners tend to realize. I was one of those original American framebuilders that went to Europe in the 70's to learn from a master. I already had a BA and MA in education and wanted to bring the secrets back to share with others. There is absolutely no way I could have found out all I did on my own. And I've continued to learn lots from others (including students) as well as from myself since then. It is absolutely inconceivable to me that someone can teach themselves as well as someone with my background could teach them.
It is not just about having the right knowledge and all the tools, a framebuilding class can really reduce frustration. I find the most necessary aspect of teaching is keeping a student from getting too discouraged. When your hands haven't mastered brazing motions and you are not sure why things are going wrong or what to do next, it is nice to have someone right there getting you out of trouble and showing you exactly what to do. You don't have to start over because you fried something and you don't have to randomly figure out the next move. There are people who enjoy the challenge of doing it all themselves and succeed but the majority need lots of help. As in lots and lots of help. In a two week class I must make over a hundred little corrections to what each student is doing so they didn't end up with big mistakes. This is after a presentation and demonstration. I often wonder what would happen if they were left on their own (but I don't intend to find out).
So I agree with Livewire that "it depends" - meaning that you might eventually make one of acceptable quality but that depends on your personality and prior shop experiences. And how fast you want to do something well. The residue of your mistakes can hang around a long time. This is not so important as a hobby builder but the stink of early mistakes can spoil a reputation of someone wanting to go pro even after they've figured things out.
Back years ago everyone that took my classes were just wanting to make there own instead of buying one. Now most have bigger ambitions. I teach 3 students in each class (with an additional part time instructor). I'm a fan of the advantages of group learning. Others ask questions you might not have thought about and make something different to learn from. In addition a presentation to a class tends to be more organized than one-on-one explanations.
While all the major framebuilding classes do an excellent job, we differ a lot in our approach. I've had a number of students take my classes after going to Yamaguchi or UBI (I took UBI’s first titanium welding class in 1992) so I know how they do things. They weren't unhappy with their experiences, they just wanted to learn more. It is a big subject. Both Dave Bohm (in Arizona) and I teach 3 day brazing classes besides our regular 2 week classes if either time and money are an issue in taking a longer class.
I want to say I am never opposed to someone learning on their own. This works best for those that enjoy the journey of discovery more than getting to the destination quickly. But I can also add they will be making an inferior product for a longer time than than if they learned from an expert.