On 8 March 2008 I wrote a short blog on Kāi Tahu Māori. You can see the complete post on this page. In that post I made two claims which I repeat here for convenience:
The letter k used to write KTM is unique in two respects. (1) it is the only orthographic symbol which incorporates underlining. (2) it is used to reference two different dialects of a language — in effect it means a [k] sound, which in other types of Māori is an [ŋ] sound.
(2) above I still believe to be true, but (1) definitely is not.
Having been trawling through Omniglot, I have come across a handful of languages which use a Latin-based script and which use underlining for various purposes.
For instance, a few Native American languages which indicate voiceless vowels this way. An example is Comanche which has voiceless versions of all its six vowels <a e i o u ʉ> (the last represents [ə]). So <a> = [ḁ], <e> = [e̥] etc.
More on this another time.