Why is a tuft of hair put on the tefillen allegedly to distinguish those following Rashi from those following R. Tam? Why tuft of hair at all as opposed to any other siman?
You are referring to Shulchan Aruch OC 32:44
קְצָת שֵׂעָר זֶה צָרִיךְ שֶׁיֵּרָאֶה חוּץ לַבָּתִּים.
A little bit of this hair needs to be visible from the outside of the compartments.
The SA doesn't explain why.
The Magen Avraham Seif Katan 61 writes:
כתב של"ה דף קי"ו י"א שהשער צריך לצאת מפרשת קדש ואני קבלתי מפ' והיה אם שמוע ונ"ל שטוב להניחם אצל פ' והיה אם שמוע בצד הפונה לפ' קדש עכ"ל:
The Shlah writes: "The hair needs to emerge out of the compartment with the parsha "Kadesh". However I have a tradition I received that it should emerge from the parsha of "Vehaya Im Shamoah", and it appears to me it's good to place the hair next to parshas "Vehaya Im Shamoah" on the side towards parshas "Kadesh"
I'm not sure where the SA got the halacha from (I don't have access to the Beis Yosef), but the discussion about where it should come out from seemingly is related to Kabbalah. First, he's quoting the Shlah, a sefer with a lot of Kabbalistic teachings. As well, the MA quotes the Zohar, saying:
שלא יצא השער כשעורה חוץ לבתים
The hair protruding from the boxes shouldn't be the size of a grain of barley
Perhaps the SA got this halacha from the Zohar. Either way there is no mention of using it to distinguish between Rashi and RT tefillin.
Nevertheless it can be used as a Siman to distinguish them, since depending on which tefillin you are using is where the hair will come out from. It doesn't seem to be the motivation, as your question suggests, yet it does end up serving that purpose.