One hack I would like to see

Basic rule of thumb, if you didn't already know, is that once the "Charge" part of the HSI fills up, you've started using friction brakes. But I think I've I've also read that they come on slightly even before it gets there, although I'm not certain that that's true.

A light would be easier to see, though, when you're busy with traffic. See page 5 of the pdf document @john1701a provides on his website. John's Stuff - Toyota Prius User-Guide

 

The Hybrid Assistant app for Android smartphones gives a visual braking indicator. It works with a bluetooth OBD adapter.
See here: Hybrid Assistant: Dashboard
Green = Regenerative braking
Red = Friction brake pads

 

Mine has 490,000 miles, Twice or more times each week I drive up 4144 feet and back down. I wondered the same thing. After 300,000 miles I’d put new pads on without turning the rotors and by chance one side makes a tick whenever the breaks are applied. Kinda like the break-wear indicator hitting something? Well from that point on I have known when the breaks pads are being used.
As others have said, until you’ve got all green bars in your HV battery, light breaking only charges the battery. Quick taps on the hydraulic pedal will bring the pads in onto the disk though continued pressure they’ll back off as the hybrid system adjusts torque between MG1 and MG2. It’s pretty impressive.
B mode uses the Passive friction of the dormant engine to keep my speed below 80 on the down grade, I was surprised to learn that even in B mode the Generators charge the battery.
My take away is that when your battery is full, you only have engine friction, break pads and wind resistance to slow you down. And that is the only time your suggested break light would stay on. If you cannot wait get a non contact IR Thermometer and do a few test runs and measure the temp of your disks!


iPhone ?

 

i believe @Mendel Leisk has reported constant friction braking with foot on pedal, increasing with foot pressure. i think he has a scan gauge.

 

Just going by ear. When the rotors are rusty (say from a wash), for the first block or two, I reliably hear a raspy sound, every time I touch the brake, lightly or heavier. Accordingly, I would say there's always some percent of friction braking.

 

Hey, RTFM! When using regen braking the system "brings the pads -to- the rotor without pressure until regen braking is not enough, then applies pressure".

Pearl S is quiet with rusted rotors until the last second or so of stopping. I'd comment on the speed but we all know regen drops out below some speed we are all supposed to remember which I forget. So I wasn't watching the speedo, just listening in fascination as the sandpaper sound came on at each stop during the last second or so (driving "gently" in the neighborhood). And it stopped happening after a few km of course as the disks got cleaned off.

I guess this means Mendel's work has tuned his brakes to the limit of fineness.

When I first joined the forum there was a guru, whose name I forget, who made a device which did exactly what the OP wants. He posted the schematics and instructions, and I was tempted to build it and put it on Pearl, but chickened out. I don't really like messing with the braking system. Especially on a Prius! With a "HOW MUCH?!!" master brake cylinder.

 

Here's another vote for hybrid assistant.
Has exactly what you are looking for.