If you know where the internal connections are and are able to attach suitable wires you might consider a bodge job (I would being a bodger).
Using the orignal fixed DC socket but external to the machine (or a replacement if you need to), firstly just soldered and taped up for proof of concept - if it works then mount the socket in a tiny enclosure for protection and run wires from the enclosure into the hole in the back of the machine.
For enhanced elegance a small in-line connector between computer and external enclosure.
The original connections may well have been plated-through holes providing connectivity to both sides of the board - some complex boards are multi-layer and can sometimes have a power-layer; like filling(s) in a sandwich, to distribute Positive and or Negative - this may be a show-stopper - I hope not. I am not familiar enough with modern mother-board construction to offer any more.
Good luck...
Peccavi
January 2010