To test the hypothesis that the subglyph N represents nasalization of a vowel, I looked at the frequency of nasalized syllables in Beaudelaire's Fleurs du Mal and compared it to the cipher poem.
I have a copy of Fleurs du Mal containing 3182 Alexandrine lines. (One poem in this copy is not an Alexandrine). Among these lines, I count 6536 nasalized syllables, so an average of 2.05 nasalized syllables per line.
If the Debosnys cipher poem is a French Alexandrine, and the N subglyph represents nasalization (and is the only representation of nasalization), then we should expect to find a similar distribution of N subglyphs in the cipher poem.
Of the 20 lines of the cipher poem, I counted a total of 30 n-glyphs, so an average of 1.5 n-glyphs per line. More specifically, the number of n-glyphs per line was distributed as follows:
0 n-glyphs: 2 lines, 10%
1 n-glyph: 6 lines, 30%
2 n-glyphs: 9 lines, 45%
3 n-glyphs: 2 lines, 10%
Among the Alexandrine lines of Fleurs du Mal we have the following distribution:
0 nasalized vowels: 366 lines, 11.5%
1 nasalized vowel: 816 lines, 25.6%
2 nasalized vowels: 897 lines, 28.2%
3 nasalized vowels: 658 lines, 20.1%
4 nasalized vowels: 312 lines, 9.8%
5 nasalized vowels: 101 lines, 3.2%
6-7 nasalized vowels: 32 lines, 1.6%
This looks like a promising match, but more work needs to be done obviously.