It is possible but I have not seen anything like this before. But we are in a brave new era with the need for new things, just like running 12+ Pi's without using separate HUBS for power.
First it is essential to understand what a a USB cable is made up of. Thanks to Wikipedia for the image, please consider donating to them.
What you are interested in is the +5V and GND lines in the USB cables. You can get a seperate (best use a regulated 5A/10A/15A power supply)
Regulated is great because the power is cleaner and stable- so the
more things you put, well, NO voltage drop basically. It stays 5V from 0% to 95%
(or 99% on better PSU's) load! Cheap USB PSU's Voltage drops on higher loads.
You determine how many amps you need by adding up the total amps of the devices connected, more or less, but this is layman's terms. You really need to read the specification and check the operating wattage because this is after spin up, and spin up wattage will also be rated and some models how efficiency mode too. Using OHM's law its easy to calcualte.
I(Current, AMPS) * V (Voltage) = P (Power, WATTS)
So a 5A regualted power supply will be happy to provide you with 25WATTS of continuous power at ~95%, allowing peak overload currents (connecting devices, spin ups, initial start, etc) without Voltage drops!
The solution requires DIY
And in your case allot of it! Chopping into the USB cable, cutting the RED +5V and stripping the BLACK GND. You don't want to back feed the Pi's (But you could power the Pi's like this too! But that is another way) It is essential the GND stays connected as GND is used to filter out noise. As pictured in these steps, 1,2,3.
LEFT THE PI, RIGHT THE HDD, BOTTOM THE POWER SUPPLY
It is also important to keep the DATA LINES shielded as much as possible, as noise will degrade or completely kill communications, so don't cut away the shield, cut into the shield, avoid cutting or nicking the data lines too! Then connect the wires and re shield the original cable, leaving on only the +5V and GND sticking out.
This can be made easier by making a PCB board, it requires no complicated IC's, just some protection and shielding. I could make and assemble these pretty cheaply but anybody with experience can do it. This could be further expanded with USB HUB IC's, where 1 Pi connects to the board and provides powered USB to 3 more device, it increases the cost slightly but you could create a massive array of transparently powered USB hubs! Then you just need to find a cheap provider of USB cables :)