generating a custom half duplex 1 wire serial protocal
It sounds like what you want is what is known as Bit Banging.  This is where you use one or more general purpose IO lines to emulate something like I²C, SPI, RS232 or whatever else you like, purely using software.

While this gives you the ultimate in flexibility, it does tend to tie up the microcontroller while any transfers are going on.

If speed isn't too much of an issue you could tie the transfer of data to a timer interrupt - transferring 1 bit every 10µS or something.

So, transferring a bit with an active low clock would go something like:


Drive signal line to level of bit to transmit (high or low - 1 or 0)
Drive clock line low
Delay
Drive clock line high
Delay
Progress on to next bit until done.