Edit: As I got a bit deeper into the copies, a new method was needed.

Common

First, make an image of the usb disk in question. To do this, open Disk Utility, and then:

1. Click File
2. Click New
3. Click “New Image From Folder…”
4. Select your folder
5. Wait

Next, find the image file in finder & mount it, record the place it was mounted.

Methodology 1

This is a variant of the work here.

Now that you’ve got the image and it’s mounted, plug in the USB hub containing your keys and run the following from your terminal:

diskutil list
$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
... snip
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *8.2 GB     disk3
   1:                 DOS_FAT_32 NO NAME                 8.2 GB     disk3s1

What you are looking for here is the first and last /dev/disk# that represent your USB keys. In my case this is 3 – 23. From there we start the copy:

for i in `jot 23 3`; do asr --noverify --erase --noprompt --source /Volumes/No Name --target /dev/disk${i}s1 & done

In the above, note the –source specifies the /Volume/No Name ## that represents where we mounted the image. What it does then, is loop over each usb disk copying the data from the image.

Methodology 2

This is a variant of the work here.

Now that you’ve got the image and it’s mounted, plug in the USB hub containing your keys and run the following from your terminal:

diskutil list
$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
... snip
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *8.2 GB     disk3
   1:                 DOS_FAT_32 NO NAME                 8.2 GB     disk3s1

What you are looking for here is the first and last /dev/disk# that represent your USB keys. In my case this is 3 – 27.

First unmount the disks:

for i in `jot 25 3`; do diskutil unmountDisk /dev/disk${i}; done

Next, use homebrew to install PV if you don’t have it:

brew install pv

Finally start the copy:

sudo dd if=/dev/disk2 |pv| tee >(sudo dd of=/dev/disk3 bs=16m) >(sudo dd of=/dev/disk4 bs=16m) >(sudo dd of=/dev/disk5 bs=16m) >(sudo dd of=/dev/disk26 bs=16m) >(sudo dd of=/dev/disk7 bs=16m) >(sudo dd of=/dev/disk8 bs=16m) >(sudo dd of=/dev/disk9 bs=16m) >(sudo dd of=/dev/disk10 bs=16m) >(sudo dd of=/dev/disk11 bs=16m) >(sudo dd of=/dev/disk12 bs=16m) >(sudo dd of=/dev/disk13 bs=16m) >(sudo dd of=/dev/disk14 bs=16m) >(sudo dd of=/dev/disk15 bs=16m) >(sudo dd of=/dev/disk16 bs=16m) >(sudo dd of=/dev/disk17 bs=16m) >(sudo dd of=/dev/disk18 bs=16m) >(sudo dd of=/dev/disk19 bs=16m) >(sudo dd of=/dev/disk20 bs=16m) >(sudo dd of=/dev/disk21 bs=16m) >(sudo dd of=/dev/disk22 bs=16m) >(sudo dd of=/dev/disk23 bs=16m) >(sudo dd of=/dev/disk24 bs=16m) >(sudo dd of=/dev/disk25 bs=16m) | sudo dd of=/dev/disk27 bs=16m

Ok, that is a single line. It is also terrible terrible terrible, but it works. Some notes:
You need a >(sudo dd) section for each disk except the last one. You will also need to change these to match your environment.