That is basically an option in 0.7 as long as the character's self control stays around 100, if you would like you can adjust it in the code yourself for 0.6 since the files aren't encrypted.
Afaik there are many minus self control foods but I am yet to find any that would increase it. Still good to know the value for it. Maybe adding such food would be an option tho honestly I would prefer a switch on character stat page to lock or even a dev/cheat mode to change the value. PY loves to assign new addresses to each value change so its impossible to do this with cheat engine it seems. As you said the only option to do so would changing the initial values from the code then starting a new game and never using minus self control food. (Tho I should check out renpy save editor too now that I know what to look for.) Besides this point, great game so I hope you would continue to develop it even further.