Hi,
I'm also sure, your request is perfect HERE. You are looking for a Mac App. (Fullstop).
This App has to download (or use an available .iso) an OS (Linux), modify or create a bootloader, and put it to a Pendrive. Exactly what UNetbootin for Windows (or Linux!) is doing.
I am also looking for such an application, without result. The "dd" will not work, because an iso is not same like a raw image of dd.
An idea is to boot knoppix (or another debian based live distribution) on ur mac, install UNetbootin for Linux, and write the pendrive.
Another way cold be WINE (using the win-version, it's a simple exe), VMWare (Cannon to little birds) or another virtualisation.
Maybe there will be a version running via mac ports or fink one day? Pls tell me when you find something...
regards,
Michael