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This might generally be true for MTB, but on a front brake only, dynamo equipped fixie I would imagine there will be lower tensile stresses on the DT as you won't be schralpin rad drops n shit. For instance ISO 4210 fatigue tests road bike headtubes with a symmetrical rearward and forward force, but doubles the forward force for MTB.
I would say go for it through the lug, but make the effort to deburr properly and seal it with paint and a grommet.
chiroshi
I saw someone recently on mtbr saying that its best to place dt holes in the upper half of the tube.
Their reasoning was that in general the forces acting on the dt/tt/ht assy are working to pull the ht off the other two. So the underside of the dt is in tension and the upper parts are in compression, as the heat tube levers off the top of the mitre.
They said holes are more likely to fail in tension and so the best bet was to put them in the top of the tube where the stresses are best suited to a hole. IDK how true this is, but it makes sense.
I think its still wise to reinforce it a fair bit as there's alot of stresses up there. And to make sure to not put it in the butt transition zone.