duus,
I think that if it is going off every 15 minutes, you'll start to get used to it and ignore it after about an hour. It is like a pile of stuff on your desk; the longer you leave it there, the more you become used to it, and it stops bothering you as much.
I don't think this kind of hack works for mindfulness.
Try this: make a hipster PDA specifically as a food journal. Write what you ate, when, and how much. Do this dilligently for a few days. Look for patterns as to what triggers you to eat the wrong things.
As for the mindfulness... maybe some randomly placed (inside the medicine cabinet, on the foods you should cut back on, in your desk drawer at work, on the steering wheel...) stickie notes with positive messages, and reminders to breathe deeply might help out.
Just some thoughts,
Grieser

What is it you want to be mindful of? Your varying moods throughout the day? How much you're eating and when? How much you're spending on certain tasks?

Are you just looking for a timer which will remind you periodically throughout the day to close your eyes, take a deep breath & relax for a few seconds?

One Zen teacher I know offered this mindfulness hack:

choose just one action that occurs regularly throughout the day to do with complete mindfulness. Anything will do: washing your hands, opening your car door, hanging up the phone. Whatever it is, just be with that one action and don't let your mind wander to the past or future for those few seconds that it takes to accomplish it.

It really wakes you up.

Then when doing this particular action has become ingrained as a mindful habit, add one more action.

That's beautiful.

Is it just me, or is the idea of "automatic mindfulness" oxymoronic? If something is truly automatic, can we be mindful of it? If we are mindful, is it actually automatic?

I like the idea, but the semantics are silly.

Actually, that's really cool in a way. like, we're so busy trying to make it easy, do it without thinking, without paying any attention and the whole point is that we're not paying any attention.

To be mindful you must pay attention.