How about "Be Thou My Vision"? It has a theme of committment appropriate for a baptism. Then there's "St Patrick's Hymn," which is in the 1940 Episcopal hymnal (not sure about the 1980s edition). Long, though.
If you like Appalachian things like "What Wondrous Love Is This," you might see if you can get hold of a Sacred Harp or Christian Harmony book -- these are more or less the definitive collections of Appalachian spiritual songs. They would fill the "very traditional" bill, if in a rather different way than a traditional Episcopalian might expect. The problem may be working out an arrangement. Some I have tried are fine with solo voice and guitar; others resist all attempts at accompaniment. A few are excellent as unaccompanied airs only (BTW the melody in those books is usually in the tenor).
Oh, there's another one in the 1940 Episcopal Hymnal called "Master of Eager Youth," to the tune of... oh crud I can't remember. It's an English folk tune arranged by R. Vaughan Williams. Works wonderfully with a guitar accompaniment.
-Glenn