Given that the fl is accessible over sysfs, you might able to cobble something together with a cmd_spawn and a chain...
Note that Nickel won't be able to track a frontlight level change made via sysfs, though. (It could, on Mk. 7 only, but I've never bothered to check if it actually did so).
EDIT: Well, it might not even be accessible via sysfs *at all* on Mk. 6 w/o NaturalLight, anyway, so, never mind me
.