I think this is a good idea. I brought it up in the firmware development thread and there were a few replies/ideas.
I think you pretty much explained the options in your second post. To do it right a direct way to sense the current is needed, and while this is possible, either using a sense resistor and amplifier or a Hall sensor, it would require hardware redesign. An approximation of constant current modes could be attained with just some clever firmware modifications, but they would be specific to a LED and/or battery combination. For example, if you wanted a constant 4A mode, with a fresh battery that would be, say, 70% PWM, then if the battery voltage fell to 3.9V, which the MCU can sense, the PWM would be raised to, say, 90% PWM to maintain 4A. This could work pretty well, but it would require custom FW for each setup.
Regarding LED efficiency, I’m not sure PWM is as bad as we think. I think the current understanding is: if an XPL produces 2000 lumens at 6A, reducing the average current to 3A using PWM would produce 1000 lumens. But the XPL produces 1250 lumens with 3A constant current, so that is a significant efficiency loss by using PWM. Well, using linear regulation is also inefficient since the voltage difference is burned off in the regulator, though it’s not that inefficient.
However, there are two conditions which cause LED efficiency drop at high current: the temperature rise associated with the high current, and the high current density itself. The high current density reduces the electron-hole recombination efficiency; I believe Auger recombination is the currently accepted mechanism responsible for the loss of electron-hole recombination efficiency at high current. I think these two mechanisms, high junction temperature and high current density, can be considered separately. In other words, looking at the graph here:
Say the direct drive current is 6A, then reducing the current by PWM would not just linearly reduce the lumen output, it would produce a curve with downward curvature because at 3A the LED is not as hot as at 6A, so it would be more efficient at 3A than at 6A. But because the current pulses are still ~6A, the efficiency is not as high as if the 3A were constant current. So the output with 3A PWM might be 1150 lumens, higher than 1000 lumens, but not as high as the 1250 lumens if the current were constant.
I haven’t seen this aspect of the LED efficiency talked about or tested.