How can I maintain the water level in a toilet bowl during extended periods of disuse?
Seriously I think this could use some technology. The idea would be to fix up an auto-flush mechanism that would simply flush the toilet once a week or so. If this was done the user flush mechanism could simply be an electronic push button to trigger the weekly flush now. 

If this was done with a small microcontroller board (i.e. Arduino for example) connected up to a mechanism that would "flip the flapper" there could be some interesting things done.


When manually tripped via the switch restart the weekly auto flush timer. 
Using the manual trip method during the day would sync the next auto flush a weeks time also during the day so that it does not flush when when you are sleeping at night. 
Manual flush is just a simple push button.
Microcontroller can log the number of flushes per month and provide data on the amount if water used.
You can add a moisture sensor on the floor next to the toilet to have the microcontroller alert for possible leaking.