OK, I had missed that thread I guess. So:

[hijack]
buttonled_dim=1

is working correctly, If I set level 6, leds go down to level 1 when headlights are on. It seems the default behaviour is not though. my usual setting is 1, so (1+1)/2 should still give 1 of illumination if I don't use that preset. In fact, whatever value I enter in the hijack menu gives me no illumination when headlights are turned on... I guess I'll use the config.ini setting for now.

Thanks for pointing out!