It was indeed a hack, but it was simple enough to strip out the existing on-screen controls, allow any touch's X value to be the player's X position, and do a cheap exponentially-weighted sum of touch movement deltas to get a 'flick strength' value. Luckily I had written my underlying 2D engine and Gundown's own UI code to be highly resolution independent, so in general things worked just fine after switching to portrait mode.
(A quick hack to test the idea. Naturally, certain things (right) broke pretty hard.)
I really like it. The metric for determining flick strength still needs tons of fine tuning, but overall I really like the mechanic involved here. In particular, I'm really excited by how much more accessible Gundown could be, since firing projectiles with simple flicks of one's finger is orders of magnitude more intuitive than the comparatively complex on-screen controls that I had been using up until now. More hardcore gamers might object, but it's my understanding that the audience that the App Store reaches is far more casual and non-hardcore when it comes to gaming. In addition, the people who I have thus far cornered and coerced into trying out the game in both its original controls and these all remarked that flicking one's finger felt far more interactive.
All that said, some measure of re-design will need to be done to figure out how to accommodate this new mechanic. Perhaps something glitzier and faster paced is the way to go, rather than the slower controlled pace that the original PC version featured.
Once this week's assignments are all cleared up, we'll see. We'll see.