You may run the shifter cable however you want, but i suspect you'll have issues with the keeping the cable in one spot, and you'll trash your finish.
The problem solvers gadget above shown at treefort will not run the cable under the BB, but it'll clamp down underneath your FD on the seat tube, and will allow you to use your existing routing, butprovide pulley-action for a bottom-pull fd.
The other problem solvers gadget will allow you to just run your cables along the downtube. You can get the little bolt-on cable guides for under the BB, and just run the whole thing like a normal roadbike. IRD makes a similar thing which, imo, looks nicer. If you're into saving money, just ask around from friends and co-ops to get an old clamp-on cable stop just like the second gadget, probably for free. I just threw a rusty one out a few months ago...
But, as everyone else has said, you can't beat the barcons for this kind of set-up. I know you like STIs, but if you ever change your mind, barcons are a l'il piece of wonderful. And, you can pick up 8speeds for under $60, new. These will outlast shimano brifters, are way more serviceable, and they just work very nicely.
-rob