I've found a workover :

def to_macroman(str)
data = CFStringCreateExternalRepresentation(nil, str, KCFStringEncodingMacRoman, 0)
if data.nil?
raise "can't retrieve data from `#{str}'"
end
dest = "".force_encoding("MacRoman")
#
# force dest to be of "MacRoman"
#
dest += CFStringCreateFromExternalRepresentation(nil, data, KCFStringEncodingMacRoman)
#____^_______ new
#
if dest.nil?
raise "can't convert data from `#{str}'"
end
CFRelease(data)
#CFRelease(dest)
#
# commented otherwise i get a malloc error
# may be it's ruby here making GC ?
#
dest.mutableCopy
return dest
end

giving the right output :

une phrase accentuée, ça vous va ?
str.size = 34, str.encoding = UTF-8
macroman.size = 36, macroman.encoding = macRoman