I have a similar question to this. I don't necessarily want different versions of PCLOS. What I want is to have different distros on a single USBKEY and choose which one I want. Is that possible?
... may be ... provided that you set up one partition for each OS, each one with it's own bootloader installed on the corresponding partition, and a master bootloader installed on MBR that will chainload each installed distro bootloader ... should work ...
Here we do that on usb sticks formatted to fat32 and installing them by hand. Usually we open a root console, mount the iso on the loop device to a directory, and copy the files to the media. Each different distro needs either it's own directory or different names for their respective directories and squashfiles. The bootloader is grub 0.97, not grub2 and the menu.list file is filled with the informations provided by the isolinux/isolinux.cfg conf file.
Tools : I know Linux Live USB for Windows, and Unetbootin for many other ones. I don't know if unebootin works well because I tried it once only a long time ago, and not again since. I was told it does.
What I don't know, is how to make a persistent boot on such hand made installs. And some of the said distros are tricky and won't install properly, or sometimes they have a hidden directory in the iso which needs to be copied as well.