What is the best way to manually choose between bi-directionally shifting 3.3v signal to 5v and "using" that signal directly?
A 'level shifter' IC, which you realize you need, would have two 'power pins'... Call them &quot;VCCA&quot; and &quot;VCCB&quot;.   So if you were shifting 3V-5V, you'd set VCCA=3V, and VCCB=5V.
To adjust what the 2nd voltage is, i.e. you want to change VCCB, you can easily do that with a 1P2T switch.    The switch would have 3V &amp; 5V feeding it, and the  'common' pin connects either 3 or 5 to VCCB power pin on the shifter IC.
Pretty simple really....
You'd probably not want to flip the switch when the device is powered. You didn't say whether or not you wanted to do that.