I2C - Can I control the SCL line with a output only pin if I am not expecting clock stretching?
Yes, for most simple slaves, master SCL can just be a push-pull output. If you feel like ensuring that a clock stretching slave did not draw too much current, you could have a series resistor.

BUT since SCL changes state 38 times to output a single byte to an I2C device, it really is the worst possible signal to put on a serial port expander.

You can share the I2C pins with other busses (see my page on that here), e.g SPI/74hc595 serial bus, or low speed uses like a status LED. So it is best to use a direct micro pin for the I2C, and share those pins with your other port expansion, or do the port expansion by I2C.

BTW, if you are only writing to an output device, then most hardware slaves can get by with SDA being an output only (since they will always respond), but it must have open drain, or a series resistor, as ACK will be pulled down by the slave.