Multiboot usb stick with Mountain Lion, Windows 7, lInux, Freebsd
OS X has no problems with MBR partitioned disks. It only checks for MBR when you first install the OS, but after that, doesn’t. If you want to be able to reinstall using the USB install disk, you can.
Just restoring the original ISOs to different partitions on the USB disk will be fine, as long as a Mac is concerned. Other legacy computers, (usually built before 2008) may be a problem.