Jasper
This may be complete tosh and I haven't time to verify it at the mo, but it may be food for thought.
If I recall there used to be a different standard for cinemas and this allowed for illuminated exit signs at all times and for maintained lighting at 10% of the standard levels during the film performance. It recognised the fact that in a darkened auditorium the audiences eyes have time to become accustomed to lower light levels and therefore can cope wth a lower lighting level.
I think that historic standard applied only to cinemas but I supose that you could perhaps make a case starting from the premise that if its acceptable to suddenly drop from say 300 lux to 1 lux on failure of the mains supply, when your eyes will have to become suddenly accustomed to a huge reduction in light levels, it may be possible to accept a lower level of maintained lighting in a darkened auditorium?
This arrangement would not cope with even low levels of smoke obsuring the routes though so maybe it would need interfaces between house lights or the full lighting package and the fire alarm system.