I wonder if the 74AHC125 chip from NXP might make sense
You still have the problem of disabling a slave's transmitter# (line driver) when the slave is not transmitting. With the new interrupt driven Serial (and lack of buffer management methods) that will be difficult.
How are you handling that now with RS-485? Or are you?
# The other three points (slave's receiver, master's transmitter, master's receiver) can be left active all the time.