After the first successful tests using capacitive sensors I implemented the thing in the chair. Here are the modules as size comparison. The new module can sit on top of 2 AA batteries and will not require more than 2 wires leaving from it.
Due to the nature of the capacitive sensor, having an ISP/serial cable connected to the PC influences the measurement. In order to develop the software I connected an LCD and in the end I thought it should stay on the chair for a while.
There are 2 electrodes made of aluminium tape: the one towards the back is the ground and the one towards the front is the sense. The capacitance is measured between the two and when a person sits, it increases. I could not find any normal sitting position on the chair which does no detect the person. Here is the whole thing assembled.
And of course, some data logging on the LCD: