For our purposes, I'm happy with the current behavior. In the mountains where we do most of our driving, having brake lights on ECO mode regen could be overkill and annoying, at least with the current level of regen. In ICE cars, drivers control speed on downgrades by downshifting, or in the case of the Prius, by using "B" mode, which does not turn on the brake lights.
On the other hand, a 20-25 kW brake light threshold might be helpful in more urban/suburban contexts, i.e., at signals and in traffic.
Adding a "B" mode, possibly with even stronger regen for downgrades, would be a reasonable way of making this selectable.
2011 LEAF at 71K miles, pre-owned 2012 Tesla S 85 at 98K miles
LEAF battery: 9/12 bars and < 49 Ah (-28% vs. new)
Tesla battery: 250+ miles of range (-5% vs. new)