Create bootable USB with multiple installers? I have a 64GB thumb drive and about a dozen or so installation ISOs I'd like to have on it.  I would like to be able to have a single partiton, so that if I boot to it I am given a GRUB-stle boot menu which will allow me to select which ISO to boot from. When I select an ISO, I want it to boot from that ISO on the thumb drive. Simple as that.

I have a 2011 MacBook Pro running OS X 10.12. What do I need to do to accomplish this task?
I'll answer each of your questions separately:

All on one partition

Not possible with the current state of technology. Apple's startup systems look for bootable partitions, not images. You'll have to partition your device if you want to put many systems on it.

GRUB-style boot menu and selecting ISO

Try using macOS' built-in Startup Manager by holding down Option on boot. This will show you a list of bootable partitions that you can boot your system from.

Installers

If you intend to put OS X/macOS installers on your partition, look into createinstallmedia. Provide the path to the installer under the option --applicationpath and the disk to put the installer on (and make bootable) under the option --volume. Example syntax as follows:

/Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --applicationpath /Applications/Install\ macOS\ Sierra.app --volume /Volumes/somevolume