I found a quote from Andrew Tridge from a while back:
you can achieve differential ailerons now using the RCn_FUNCTION settings
setup (for example) both chan5 and chan6 as ailerons. Set their RCn_MAX and RCn_MIN to the throws upwards are more than the throw downward
I think you could also use KFF_RDDRMIX to automatically mix in rudder to aileron to counteract adverse yaw.