"o", "u", and "u"

[John Sullivan]

I am working on a nahuatl document from los altos de jalisco whose writer
seems to distinguish between the "o", its "u" variant, and the "u" of "cu"
and "hu". The "u" in "hu" and "cu" is written with the normal letter "u" (it
has a little tail). The "u" which is a pronunciation variant of the "o" is
written with a letter "o" that is open at the top, kind of like a "u"
without the tail. The "real" "o" is written with a normal closed letter "o".
Has anyone seen this fenomenon before? Is it documented? Also, when I get
this published, I would like to distinguish between the three sounds in the
transcription by using three different letters. Can a linguist give me
advice on an appropriate symbol for the "u" variant?
    John Sullivan
    Universidad Autónoma de Zacatecas