After much Googling, I finally found the solution, posted in some newsgroup somewhere (sorry, I didn't mark the page): create a simple shell script that repeatedly runs the task, with a sleep command between each call to the job. In my case, my shell script looked like this:
#!/bin/sh
open /path/to/getpic.app
sleep 15
open /path/to/getpic.app
sleep 15
open /path/to/getpic.app
sleep 15
open /path/to/getpic.app
sleep 15
This will run the task four times over the course of a minute (not allowing for processing time). I then created my cron entry to call the above script, with the minute interval set to "every minute" (*/1), and sat back to see what would happen (not for the full twelve hours, of course!). The final result? Not too bad, but not perfect -- I captured a frame about every 25 seconds, instead of every 15. I think I need to reduce the sleep value to allow for the processing time of the getpic.app, so I'll try that later today. As for exactly what it was I was capturing for 12 hours, well, that too is for another day. (It will probably show up on my blog, not here -- though I may write up the 'acquire image' bit as a separate hint, as it was a little tricky.)
In summary, if you need a cron task to run more than once a minute, create a simple shell script using sleep, and have it call your other task the required number of times per minute, then tell cron to run that shell script. I'd love to hear if there are better ways to do this (via cron, launchd, or any other mechanism...).

