If the .iso contains all the right software, correctly configured, to boot a Mac a flash drive (see note at bottom of this Answer), then you can do this:

1. Convert the .iso to a .dmg:

   hdiutil convert -format UDRW -o filename.dmg filename.iso

2. Connect your USB drive to your Mac.

3. Make sure you've got all the data you care about off of all partitions/volumes of the entire drive.

4. Use the output of this command to figure out which disk # your USB drive shows up as:

   diskutil list

   You're looking for an identifier like /dev/diskN.

NOTE THE NUMBER AND USE IT IN PLACE OF N BELOW.

5. Unmount all volumes from this disk to prepare it to be overwritten

   diskutil unmountDisk /dev/diskN

6. Use dd to write the .dmg to the flash drive (note that we need the "rdisk" version of the disk device for this command; don't forget the 'r'!)

   sudo dd if=filename.dmg of=/dev/rdiskN bs=1m

7. Eject the disk so we can use it somewhere else:

   diskutil eject /dev/diskN

Note that if your .iso is set to boot like an ISO9660 CD-ROM instead of a hard drive, there's a Perl script called "isohybrid.pl" that you can run against your .iso to make it capable of hybrid booting (like either a CD-ROM or a hard drive). So you may need to run that against your .iso before you do any of the steps above.

I have successfully performed these steps on a "System Rescue CD" bootable Linux .iso and booted MacBooks from it.

If the .iso contains all the right software, correctly configured, to boot a Mac from a flash drive (see note at bottom of this Answer), then you can do this:

1. Convert the .iso to a .dmg:

   hdiutil convert -format UDRW -o filename.dmg filename.iso

2. Connect your USB drive to your Mac.

3. Make sure you've got all the data you care about off of all partitions/volumes of the entire drive.

4. Use the output of this command to figure out which disk # your USB drive shows up as:

   diskutil list

   You're looking for an identifier like /dev/diskN.

NOTE THE NUMBER AND USE IT IN PLACE OF N BELOW.

5. Unmount all volumes from this disk to prepare it to be overwritten

   diskutil unmountDisk /dev/diskN

6. Use dd to write the .dmg to the flash drive (note that we need the "rdisk" version of the disk device for this command; don't forget the 'r'!)

   sudo dd if=filename.dmg of=/dev/rdiskN bs=1m

7. Eject the disk so we can use it somewhere else:

   diskutil eject /dev/diskN

Note that if your .iso is set to boot like an ISO9660 CD-ROM instead of a hard drive, there's a Perl script called "isohybrid.pl" that you can run against your .iso to make it capable of hybrid booting (like either a CD-ROM or a hard drive). So you may need to run that against your .iso before you do any of the steps above.

I have successfully performed these steps on a "System Rescue CD" bootable Linux .iso and booted MacBooks from it.