I believe there are a variety of crossfading curves in use for mixing, generally. Perhaps a formula device tied to master channel fader would do the trick?
Ooh, or you could make a doofer with a gainer on the master channel, min -inf, max 0 db, then set the scaling curve to logarithmic. Invert min and max values on the second instance and tie both macros to the same midi mapping. Haven’t tested it, but should do the trick, I think