A 405x mux on SDA should work fine. A start condition in I2C is a high-to-low transition on SDA while the clock is high, and this can't happen if SDA is disconnected. So, no SDA = no bus transactions.
Clock stretching is rare on 'real' I2C slave devices, but it does get used if the slave device is a microcontroller. It takes time for software to recognise the slave address, look up the proper result and load it into the outgoing I2C shift register, and if it's a slow processor, the clock must be stretched.