dev-qt/qtgui (5.7.1-r1) has a libinput useflag. I think it would make more sense to use the INPUT_DEVICES expansion for this, which already contains libinput aswell.
Does not sound that bad but I don't believe we really need this. Not all Qt input options are currently mapped to INPUT_DEVICES (tuio, tslib), and generally I tend to think this particular USE_EXPAND is not all that useful, unlike e.g. VIDEO_CARDS. Majority of users will just leave the profile default alone nowadays. Also, input on Qt is not as important as it sounds. Even if you disable all devices (libinput, evdev, etc... -- in Qt6 this is allowed now), you'll see be able to input fine because it's mostly handled by X/wayland unless you're using eglfs or so. Could revisit if we really wanted to streamline these, but for now I'll close this.