metal banister idea :)

why not a weight activated switch, the switch could be actuated by a metal plate that sits flush with the floor, the plate could be covered with a mat. When someone walks over the plate, the switch gets activated.

You may have to do some modifications to the house, but if the plate and the switch assembly are slim enough then you could use it straight away without making it flush with the floor.

Just my thought

You may be able to use this circuit using a 555 timer for a touch switch:

**broken link removed**

This may work. The only thing I can think of that might cause a problem is that it might be triggered by mains "buzz", but I think the transistor might reduce the risk of that happening.
Hope this helps, and good luck!
Der Strom