After talking with 2 Toyota master techs at 2 different dealerships, here is their consensus. One can assume that Supras are under the heavy duty usage maintenance schedule since most Supra owners like to understandably push their vehicles to the limit. Nevertheless, if the Toyota Type-T IV ATF fluid is not contaminated (brown) and/or smells burned, it is not necessary to perform a flush service at the 15,000 mile interval. A drain and clean pan and screen service will suffice. Even more interesting, it was recommended that if the fluid is drained via the plug, and the tranny topped off with the necessary 2 replenishing quarts at engine oil change intervals (~2500 miles) then that is sufficient to introduce enough new detergents and additives so as to not require an eventual full flush. In fact this 2500 mile regularly scheduled simple drain may even be of greater benefit to a longer lasting tranny since the unit does not go extended periods of time between seeing fresh detergents.
A few footnotes though. If you're a new owner and not familiar with the previous owner's vehicle maintenance, then an initial flush is recommended followed by the 2500 mile drain and replenish schedule. If you have to flush the fluid you can easily reproduce the dealer's flushing technique in your home garage. Simply disconnect the upper hose going to the transmission oil cooler at the radiator and allow this hose to drain into a bucket or suitable container. Start the engine and allow it to run until 2 quarts of fluid have drained into the bucket. After 2 quarts have drained, stop the engine to stop the draining. Empty the bucket. Pour two quarts of fresh Toyota Type-T IV ATF fluid into the dipstick via a thin stemmed funnel. Repeat this 2 quart drain and fill cycle until the fluid coming out of the hose is the same as the new fluid. The tranny capacity is approximately 8 quarts but the flush may require 10-12 quarts to thoroughly freshen the tranny's entire capacity. Reinstall the hose and check that the tranny fluid level is up to spec.
I'm sure I've left out a few details. I welcome all feedback.