Yup. With a gun to my head very persistant enthusiast, I'd try drilling holes in the top tube - at least 5 cm away from the welds, just in case.

Thinking out loudly (haven't ever done something like that... drilling for internal routing I mean... not that I'm exceptionally good at thinking either ):
The next challenge would be preventing cables from slowly cutting through aluminium. Full housing routed through the frame would prevent that, but would also provide a bit more flex (not a good idea for both the brakes, and the shifters).
Alternatively, some sort of steel reinforced housing ends/stops that guide the cables on entry and exit could do, but if they are permanently fixed, they would also make cable replacement a bit more tricky (in either case one should try to use the existing cable as a guide when replacing the internally routed cables - whenever possible).

I also believe spare plastic cable/housing guides for frame entry/exit are sold somewhere on them Internetses and/or bike shops - and I wouldn't be surprised if every model of every manufacturer uses a slightly different mold (guide shape, and frame hole shape/size), but I could be wrong on that.

If done right, it will definitely look "smoother" when done. And any future cable/housing replacements will most likely be a lot more hassle (but also quite meditative ).

While I'm at it, at the cost of sounding like a fear-monger, I'd advise against drilling the fork, stem, bars, or the downtube. Just to be on the safe side.

BTW, if you go through with this, I'm interested to see what you opt for eventually, as well as to see the pictures, and hear how it went in the long run (i.e. the next cable replacement).
I'm sure you aren't the only person who prefers the look of the internally routed cables so it would probably be useful to any other enthusiasts.
I still think it's a bad idea (just like the 1x drivetrains and hydraulic disc brakes on road bikes ), but it's also interesting.