Allow Lux to average/smooth/integrate the readings as it does today (with jitter control and dynamic response delays), but once Lux does decide a target screen brightness to go for, have it do that slowly (i.e. let the user set the "Fade brightness changes" setting to a quite high number of seconds, and maybe the number of steps). And maybe ignore this 'inertia' setting the first few times after the screen wakes up so the brightness is about right quickly when you first wake up the phone in new ambient lighting conditions.
I'm thinking the user experience doing that may be a good complement to the settings that smooth the target screen brightness Lux goes for, when the brightness changes frequently... In my car (using sat nav near the windscreen) I still struggle to get the best Lux settings for when the car goes under street lights, and headlights from other cars shine through the back window on to the phone's sensor. I feel that if only the phones screen brightness would change even more slowly the experience would be a lot better. (I would try small dynamic response delays and higher 'inertia' to better smooth out the effect of temporary changes in brightness)
And in a perfect world, Lux's Locale plugin support would also allow a setting for 'inertia'
Hope that make sense.