I vote for this feature, too
A lot of us guys use osmand+ for cycling using gpx-tracks. a long ride lasts some hours and there is no way to recharge.
just use the simple algorithm
being in bike-mode:
no routing in e.g 1000m -> turn off the display or better just turn off the backlight (depends on power consumtion). When there is routing message spoken (and the target is just 200m or less ahead) just turn on the display and leave it on for e.g. two minutes.
this will save a lot of energy and let me finish my ride. Turning the display manually is cumbersome due to lock-screen