I will agree that the vast majority of cases have considered labor to be taxable, however, I have yet to see a law that clearly says so. The closest I have seen is what you pointed out earlier when you cited to Title 26.
My area of question is the assessment of profit/loss. Making such an assessment involves calculating a starting value (not necessarily a purchase price) and the selling value. You posit that the starting value is 0, simply based on the idea that nothing was paid to another person for the labor. I question if this is accurate. When calculating the base price of something you have to consider all the expenses involved; for example, if a company manufactored a chair and sold it for $100 dollars, they would first add up the cost of the wood, nails, sandpaper used, amoritize the buildings and machinary used, etc. These expenses would be subtracted from the $100 dollars and what was left would be the profit, aka, taxable income. In the exact same manner a similar trade occurs with labor. Instead of wood and nails being expended to create the finished product of a chair, time and energy are expended to create the labor. These elements do have a value which can be calculated. In the same way that a business assigns a value to labor and expenses for the chair, so too could an individual expense their time and energy against their labor. To calculate the value of those components one only has to look at the fair market value of it, subtract that from what is gained in return for it and you would be left with the resulting profit (income). This would probably be 0 unless someone is unreasonably charging above market rates.
I'm sure that some would disagree with the above argument, however, it does seem to be in line with Generally Accepted Accounting Principles which is all that's necessary for tax purposes and income statements.
If you were wondering, yes, I too am one of those who gets to "eat, sleep, drink, and piss" law....speaking of which I need to get back to pulling my International Business Transactions outline together, interesting discussion though.