Although probably not what the OP wants, there is a third party power supply that brings the USB charging receptacle out to a few inches away from the main tablet power connector rather than on the brick like the OEM power supply. Here is the car version:
http://www.amazon.com/Nuosi-Deng-Po...r=8-2&keywords=surface pro 3 power supply usb
And the mains/wall-plug version:
http://www.amazon.com/Nuosi-Deng-Po...ref=sr_1_2?s=pc&ie=UTF8&qid=1420227205&sr=1-2
OEM version is like this:
=------------[||-usb-||]----------------------------------------tablet
3rd-party is like this:
=------------[||||||||||]--------------------------usb----------tablet
=======================
As for using the 5V USB source to output 12V to charge the tablet, you need a 5V->12V DC-DC boost step up converter of which many are available from electronics suppliers on
ebay, aliexpress, dx.com etc. Just search for "5V boost converter" here is a good example:
http://www.ebay.com/itm/30W-DC-5V-t...034?pt=LH_DefaultDomain_0&hash=item3cdf0a0ea2
Some like this one have a preset voltage. Others have an adjustment screw to change the voltage so you need a multimeter to set it right and you'll want to check the voltage again during charging. You'll need to supply the USB connector to solder on the board as well as a generic 12V barrel-style output receptacle and a generic Surface Pro 12V charging cable as mentioned in many other posts on this forum. Finally you'll probably want to put the whole thing in a small plastic electronic project box.
One issue you'll run into is that most USB will only output 5v@ 2.1amps or less. That's only 10.5 watts. The Surface Pro 3 charger is rated at 36 watts so it may not work at all. I know the older Surface Pro 1 and Surface Pro 2 would accept a lower current charge from the smaller Surface RT chargers and just take longer to charge. I'm not sure about the Surface Pro 3 - it may reject a lower current. If you had two USB 2.1amp receptacles could you run them in parallel as an input to the boost converter? Maybe - it depends on the configuration of the USB outputs. It would be a fun project, but probably not worth the trouble.