We use figured bass notation to help indicate chords in inversion by establishing the interval between the bass note and the other notes of the chord. I have seen figured-bass symbols applied to major triads, resulting in the "five-three" chord (root position), the "six-three" (3rd in bass), the "six-four" (5th in bass), as well as to 7th chords, giving us the "seven", "six-five", "four-three", and "four-two chords". Forgive me if this explanation is too brief or if you prefer different nomenclature.
For analysis, I prefer using roman numeral notation instead of "slash" notation, in which the chords are named . . . by name. For example, a I63 chord in C major would be written as "C/E". Now, depending on the song, there may be chords that have neither non-chord tones nor the 7th of the chord in the bass, such as a C major chord with a D in the bass. In slash notation, this would be a "C/D". How would this be written in roman numeral/figured bass notation? Note: please treat this C/D example, along with any chord with scale degree 2, 4, or 6 in the bass, as an actual chord, rather than a chord with a counter-melody moving in the bass line--I am not thinking of the bass as a passing tone here.
Is there a procedure for naming chords with scale degrees 2, 4, or 6 in the bass? Please let me know.