There are 2 ways you could do this:
- Skip the bootloader and run the ArduinoISP sketch on the Leonardo
- Use the Leonardo as an USB-to-serial interface and program the 328 via the bootloader as normal
The second option is probably easier, but entails writing a short sketch for the Leonardo to link Serial and Serial1.
I think option 2 is a little more involve (though I do like the creative solution) such as the bootloader. If you upload the sketch to the Leonardo, it is going to hit UART0 because of the bootloader and either reprogram the Leonardo or hit UART0 and find that the 32u2 is the wrong chip. If the bootloader is overwritten, I am not sure the USB2TTL will work.
My vote is option 1, but it will overwrite the bootloader on the Uno.