I need a wireless gadget that may not exist. Pic and description attached
I think I would just fix the problem.
 By this I mean add a point of use water heater usually at the shower.
Most of the time with on demand water heaters I have found most customers want more hot water for the exact same reason you have. 

Adding a 30 amp 240v on demand water heater brings the tepid water temp to the nice warm water most home owners want, yes even when there are other demands that drop the water temp 15-25 degrees in the shower.

It is all based on flow but adding a small electric point of use electric water heater is a inexpensive way to have almost endless water at a comfortable water temps.

I can personally say this method works having a home with 3 full baths and 3 daughters , and up to 6 granddaughters at the same time, I started with a point of use electric in the master bath, the next year in the upstairs bath and before we bought a different home the 3rd bath. Now even with the entire family home the main water heater a gas on demand warms the water and the point of use electrics bring it up to temp even with all 3 showers being used at the same time. 

I loved the gas tankless but it did not have the flow needed for even 2 showers at the same time, after adding the point of use electrics on each of the baths the power bill actually went down and everyone could shower back to back without running out of hot water and the space saved by removing the main 100 gallon water heater provided more room. 

Having a point of use solves the same problems with a tanked or tankless hot water supply, I have added similar small heaters for other customers and most have really thought they did the job. 

I would consider an electric point of use tankless on the shower. It may be the solution you need.