Voynichese is a numeric cipher?

[ololololo]

The roman_rules list doesn't have entries for 7, 8, 13, 14, or 18 - is that on purpose? Would you need those added ith they glyphs in descending order?

Yes, this is intentional. 
7 - VII - ain, 8 - VIII - aiin; 13 - XIII - eiin = daiin (d and a are V. VV is equivalent to X); 14 - XIV - oke/ote (actually eot-eok, but I changed it); 18 - XVIII - oiin.
If I add a glyph for each number, I'll end up with a regular substitution.