Hi everyone,
I am about to flush the brake system on my 2003 GS300 and am wondering if the job can be done by one person with a Power Pressure Brake Bleeder attached to the master cylinder cover.
This way, I can control the laptop with Toyota TIS and perform steps there, including periodically opening the bleeder valves on the calipers without depressing the brake pedal.

You don't need a power bleeder for the GS300. It has built in pressure pump and accumulator on the ABS system. You will need a one way check valve on a bleeder bottle like this (Allstar Performance ALL11017 Bleeder Bottle) from Amazon. Just turn the key on to accessory to start the pump and pump the brake pedal a couple of times.

Will that work for both the front and rear? I thought the front brakes had to be actuated with Techstream...

Yes it will work for front and rear. That's how I bled my brakes by myself front and rear. My pedal is nice and hard.

This is what I was thinking.
The service manual says to activate the front brakes with Techstream and bleed the rears by turning the ignition on, opening the bleeder plug and lightly depressing the brake pedal. This will turn on the pump and bleed the rear system.

There is a possibility that the front brakes can be bled in the traditional way by pressurizing the master cylinder, either by pumping the pedal or pressurizing the reservoir, because there is an emergency mechanical system in the master cylinder that works without power to the ABS unit, the pump and VSC valves. It probably will not bleed the ABS unit internals because the solenoids will be closed inside.

Thanks again for that suggestion. I finally got to bleeding the brakes this weekend and it was a breeze. I waited way too long to do it...