It took me a while to get this far and I'm even further away from being done. I couldn't find a good example how to use the I2C library for it. http://wiringpi.com/reference/i2c-library/

As I understand now, the function wiringPiI2CSetup gives you a file handle (everything in linux is a file, right?) and you need to store it! That's what I'm doing with the fd variable. Ignore the broken math, it's what I got from 4 hours raging on the keyboard after hours of work. From there you can use the fileHandler thing to read and write from and to. This way I got some data from a 4x4 keyboard.

Little console helpers:

i2cget -y 1 0x20
i2cset -y 1 0x20 0x87
gpio i2cd

I also messed up the PCF8574 addressing pins :D of course I did. Here's my messy script.

#include <wiringPi.h>
#include <wiringPiI2C.h>
#include <stdio.h>

int fd;
int rpl;
int row = 1;
int col = 16;

int main (void)
{
        fd = wiringPiI2CSetup(0x20);

        wiringPiSetup ();
        for (;;)
        {
                if (row>8)
                {
                        row = 1;
                        col = col << 1;
                        if (col>128) col = 16;
                }

                rpl = wiringPiI2CWrite(fd, 240-col+row);
                int b = wiringPiI2CRead(fd);
//              printf("\t%i", 240-col);
//              printf("\t%i", row);
//              printf("\t%i\n", b);

                if (b != 240-col+row)
                {
                printf("\t%i", b);

                        printf("\t%i\n", 240-col+row);
                        delay(250);
                }
// delay(250);
                row*=2;
        }
        return 0 ;
}