Combining latin small letters with diacritics

Hi,

Could the following be decomposed instead of being encoded as single characters?
COMBINING LATIN SMALL LETTER A WITH DIAERESIS
COMBINING LATIN SMALL LETTER O WITH DIAERESIS
COMBINING LATIN SMALL LETTER U WITH DIAERESIS

The phonetic alphabet of Gillérion and Rousselot used in the ''Atlas
linguistique de la France''[1] and several other French dialectology
texts use things like combining i with tilde, combining o with breve,
combining o with tilde, combining oe with breve.
Isn't the shape of the diaresis, tilde, breve or other diacritic
contextual here?

[1] pp.19-24 http://www.archive.org/stream/atlaslinguistnot00gilluoft#page/18/mode/2up

-- 
Denis Moyogo Jacquerye