I thought that a badly tuned Geo Metro was a 3 cylinder with cylinder de-activation (albeit randomly due to misfires)
Jokes aside, that is how I would do it - randomly miss an injection cycle every 3-4 engine revolutions and cycle through which one was missed in the reverse of firing order. This would increase the load factor and slightly decrease fuel usage without much work. The lambda sensor will probably not be affected too much.
Simon