Activate app by pressing thumb on timeout / locked screen for few seconds
Without jailbreaking, no. Even with jailbreaking, quite possibly not (you'd likely have to keep the entire screen on, which would burn a lot of power). The capacitive screen is turned off until you wake the device, so no amount of touching it will do anything.

Additionally, there's no real way to "calibrate" for the owner's thumb: a touch is a touch (on iOS devices). Objects in your purse or pockets, though, are rarely noticed by the screen because it's detecting energy being drawn away from it by the touching object (your human body is serving as a capacitor); that's why you can't register a touch with, say, a plastic pen or a gloved finger: the electrical charge isn't being siphoned off because it can't be transmitted through the pen or glove. Nothing commonly found in a purse or pocket would serve as a capacitor, so that part's safe, anyway.