Yeah not a bad idea… I’m not sure how you would go about finding springs with the exact right tension and rigging up some kind of platform though.
This post in another thread looks like a perfect device to use: Devices we don't have
It seems to be able to determine the level of pressure applied so one could, in theory, determine the amount of food or water left in the bowls.
Unfortunately it looks like the Kickstarter campaign was not funded though: https://www.kickstarter.com/projects/1340651422/smartmat-wi-fi-floor-mat-that-can-alert-mobile-dev?ref=discovery