Early Rules

Below is an example of the earliest rules written.  These were what I gained from listening to the few resources I'd examined in the later months of the pre-production page.

I went over these rules with my supervisor at the time and over time learnt what worked and what didn't work in the context of the project.  Some of these rules survived to make it to the end of the project - others were removed when it became clear they wouldn't be that useful.

Major triad chord can be replaced with Maj6add9
If there exists more than one 4-5 introduce a ii-V in the key of the 5 chord every second time.
I-IV - add minor ii-v in relative minor key of IV.  Eg. F-Bb play F- Adim7 - D7b9 - Bb
If V-I add ii before V OR VII9 before V (the latter is the tritone sub of the ii)
Root chords that do not start or resolve a line can be replaced with a vi
Said vi can then include a ii-V sequence
The ii chord can be substituted for a ii#9b5 chord (if going to a ii-V especially)
If a I chord begins a chorus, add a V7 before it.
If going to a V7, on each beat of the preceding bar back cycle into it (eg. if G7 play | B7 Bb7 A7 Ab7| on the bar before it.