here you find some background to this bug/missing feature: https://bugs.launchpad.net/mixxx/+bug/355945
If you are going to implement it, for the control I'd avoid any automated system like the signal quality, just put a button on the main interface next to the waveform as it's done in Traktor Scratch. Don't hide it in some menu, it's something that will be used frequently so I think the best place to control it is from the main interface on the deck as Traktor does.
See the attached pic: the yellow "thru" button on deck 1 is what Traktor uses for this purpose. On the deck 2 you see a grey B. By clicking on the yellow thru or grey B you can change from timecode to pass thru and viceversa.
Regarding the signal quality, bear in mind that if for any reason the quality of the signal degrades while the record is playing on the main output of the mixer and the software switches to pass through mode then the crowd will hear a loud whistling noise and the DJ will look like a giant a**hole
Regarding the pitch: only the turntable/cdj should control the pitch, no control from the software, it would create confusion. In Traktor the pitch control is automatically disabled when in timecoded mode, you can only do temporary pitch bend (like nudging the record) from the software. The DJ that uses timecoded vinyl is generally not keen on touching the computer too much, so most of the action would take place on the turntables/CDJ and external mixer, the laptop would be more like a big box full of music, effect generator and sync-helper.