Hello
Here is a simple touch switch, with only 3 components: a darlington transistor (or 2 transistors), a 4.7uF capacitor and a 22k resistor
It work find with the sketch "Button".
So, with a PCF8574, an ULN2803, a resistor array and capacitors, you have a real lowcost I²C touch pad 
PS: Sorry for my english, I haven't practiced for 8 years..