I have owned Toyotas that had drain cocks on the block. I don't know if the Tundra does or not. The coolant replacement section in the related Toyota Repair Manual specifically instructed draining of the block. I expect Toyota has continued that practice with later cars and trucks, but I won't know until I dig into it. When the time comes, I will drain the block if possible.
As for flushing the cooling system with tap water, then using coolant concentrate and the remaining water in the block to dilute, I would not. Tap water, depending on it's quality, has more-or-less minerals in it, and eventually those minerals can precipitate in your cooling system. Distilled or de-ionized water is cheap. If there is a price advantage to using Toyota long-life coolant concentrate (I assume it's available as concentrate), I would use that with distilled water. If Toyota's long-life coolant is only available as 50/50 mix, then I would use that believing that Toyota's water was distilled or de-ionized.
Current Toyota recommendations for the Tundra are to "inspect" Toyota "long-life" coolant every 15K miles or 18 months (Mmmm...Yep, looks like antifreeze to me.), and to replace the coolant at 100K or ten years. This is an extremely favorable recommendation, but I'm sure they have a solid basis for it. If you follow their "new truck" recommendation, I see no reason why you cannot repeat that recommendation and experience again, i.e. another 10 years with Toyota "long-life" coolant.
Finally, water and ethylene glycol may never wear out, but coolant does. All coolant, especially those products intended for aluminum engines, have corrosion inhibitors in the mixture to prevent corrosion. Pretty gutsy to intend to use the same coolant for the life of your aluminum engined truck...but then, the life of your aluminum engine might be shortened if old coolant isn't preventing corrosion. Your truck; your choice.