Seems that detecting parts on the nozzle will be quite easy.

First section of the graph is with closed vacuum valve - sensor is on atmosphere pressure. Then valve is opened (no device attached to the nozzle )and ADC value goes quickly down to 3300. After device is suctioned to the nozzle, there is another drop down to 2900.

This test was performed with 0.9mm nozzle. So for smaller nozzle diameter (for placing 0402 packages) difference in pressure will be much smaller, but still be detectable.