J.T.,
Welcome to the forum! I'm a reenactor too: NY Provincial for F&I War, and also 1745 Jacobite. What you are saying re: plaids and philabegs has been "standard" knowledge for years.
However, some recent research and conjecture regarding the belted plaid is that it likely had a drawstring, running through sewn loops at waist level, to ease the donning and wearing of the cumbersome item.

Again, consider these two articles on the subject:

http://www.tartansauthority.com/Web/...eltedplaid.asp

http://albanach.org/drawstring.htm

So, my question remains: if the plaid had a drawstring, would not also the philabeg when it was split away from its wider forebear...?