Intervalic Chord Names

[Tom Swan]

Hello. (Trying this again in plain text. Sorry for duplication but my previous 
try didn't seem to go through.) 

I need some help figuring out how to change chord names (C, F, G) into 
intervals such as (I, IV, V). The short example below displays a timing diagram 
for a melodic line, but I would also like to display the chords C7, F7 as 
generic intervals I7 IV7. Manually setting the text would okay -- I'm not 
looking for some kind of automatic translation, although that would be neat. I 
know I can do this with markup, but I do not want to add markup statements to 
noteValues! Do I need to set chordRootNamer somehow? Or is there another 
solution? Thanks for any advice.

\version "2.18.2"

globalDefs = {
  \clef "treble_8" 
  \key c \major
  \time 4/4
}

noteValues = {
  a4 a4 a8[ a8] a4 |
  a4 a4 a2 |
  a4 a8[ a8] a4 a4 |
  a2 a8[ a8] a4 \bar "|."
}

chordValues = \chordmode {
  % \set chordRootNamer = ???
  c1:7 f1:7 c1:7 c1:7 
}

\score { << 
  \new ChordNames {
    \chordValues 
  }
  \new RhythmicStaff = "Timing" <<
  \new Voice = "Rhythm" {
    \globalDefs
    \omit RhythmicStaff.Fingering
    \improvisationOn
    \noteValues
  } % Rhythm Voice 
  >>
>> 
}