I'm probably going to get yelled at for saying this :brownbag , but theoretically,you could do it with the right amount of care, the right tools, and a strong shop vac with attachments. The biggest problem is making sure that shavings don't find their way into the runner. They MUST be removed/sucked out entirely. The easiest way I can suggest is to remove the upper intake manifold, so the vacuuming part will be easier. You can pack the NPT tap with thick grease to hold on to the cuttings better, too.
Okay, now I'm thinking about the circumstances you're going to have to do this in: in the vehicle, under the hood, with a few progressively-larger drill bits, and tapping an NPT with a pipe tap and tap wrench while you're standing on top of the engine. :shocked
God, even after all my experience with tools and such, I'd be afraid to do it this way. :scratchhe If it were MY vehicle, I'd remove the lower intake just to be sure nothing went into the intake port, and do the tapping while the intake was mounted in a vise or jig. Thread-tapping ain't no game; you have to be square and true, especially in aluminum.
Let's take another look at this problem. The ACT is primarily used for timing functions and the related 'tweaking' adjustments by the EEC. Typically, higher temperatures make the EEC pull timing advance back to prevent detonation. If you just left the sensor on top of the manifold where it is, it'll likely get some heat from the surrounding air and manifold. If you live in a climate where the temperatures can be extreme, such as here in Michigan, the desert, or the mountainous regions, I'd suggest the aforementioned removal of the manifold and tapping. However, if there's little temperature variance where you live, and being the lazy-ass that I am, I'd just make sure it's functioning properly by itself, and leave it sitting there. If you ever notice spark knock, or you're one of these people that run their ignition timing right to the ragged edge, I'd again suggest getting it installed in the runner.
Hope this helps.