Pressure/force map (0-5kg) via sensor grid
This may be possible using quantum tunneling composites (qtc's) which use tunneling effects to drastically change in conductivity with applied pressure. You could use a grid of multiplexed electrodes on the back to "address" a particular spacial location of the qtc mat and measure its resistance. This would give you a grid of force data that could then be 'blob' tracked similar to image recognition algorithms. Once the 'blob' that is the object has been identified you could sum the forces contained within its boundaries to find the total weight. Similar things have been done in industry (http://www.nanotech-now.com/news.cgi?story_id=47663)

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Accuracy will depend on the dynamic range of your ADC circuitry as their resistance can vary on the order of 10 orders of magnitude depending on the pressure. I can imagine a digital capture circuit that uses feedback that selects the optimal reference voltage or voltage divider resistor. However, I believe your 1% accuracy requirement is far to high most technologies.