Regarding "food seasoning". Our tongues sense dissolved salt: if you could make the plastic release it's constituents only in the vicinity of the taste buds then it would be very cool, but also very difficult (unless you are prepared to eat a lot of dry food).
The salty taste sensation is transduced through ion channels (Na+ and Cl- are the chemical species that trigger the response) so a salt substitute would more realistically have to be a small molecule. Allosteric modulators/activators of the chlorine channel would do the trick: you'd add it to your food to make the salt that's already in it (humans need a gram or two a day to live) taste more salty.
Such a "taste intensifier" would probably have to be degraded fast (as it reaches the stomach, for example), before it can enter the blood stream, since normal operation of chlorine channels is vital for a wide variety of essential processes.
Fat/oil substitutes are already out there (Olestra, for example). These are simply hydrophobic molecules that are not ingestable and pass through you without any caloric contribution (even silicon oil could be used). One of their drawbacks is the "slippery anus syndrome"... Alternatively, the weight loss pills (containing, for example, chitosan) that bind normal fats and prevent it from being ingested can also be used: these have the added benefit of dietary fiber but sadly only bind a fraction of the fats you eat.
