I believe it can be done, safely.. But it would take some research and some analysis of your bar. But honestly, if it's steel, I think you can safely get away with it. For the most part, stay as far away from the stem as possible, as you will find this is where there is the most stress.

If it's a risk you're willing to take, go for it. Someone had to take this risk to begin with, right? (for there to be internal cable routing at all, and then I'm sure they tested it, and either stuck with it, or improved on it). If you're not willing to take it, a set of bull horns with internal cable routing can be had for not much money at all.