This can be done in Intervals.icu. First you need to add a custom activity field “Max BR” to the sport in /settings. The value will be copied on to activities for the sport when they are analysed (like FTP etc.).
Then you can use max BR, the respiration and heartrate streams, and the athletes max HR to create a custom activity chart using Javascript and Plotly.
Or you can use max BR and the respiration stream to create a custom activity stream “BRR”. This can be plotted on the activity timeline with the existing HRR% stream on a custom chart. This is probably easier than Plotly.
But I think we will probably add “Max BR” (and BRR%) as built in things soon, so wait a little bit.
Edit: You also need “Resting BR”.