Oh man you're having me cry right now, this would be so damn amazing.
However, reading this bit vvvv
-Scripts for SP missions would be disabled when playing as the custom character. You would still need to switch to one of the SP protagonists to play them.
This makes sense to have happen, but to stop this from happening, you could create voice lines that you wish to have your custom character use (E.G: Carl Johnson from SA: He has screaming sounds, pain sounds, voice lines (either when stealing a car or attacking someone, or in story missions.))
However I feel story missions would take longer to do voices for. Seeing as how a few thousand lines or more would have to be read, and that could take days, weeks, months, years, generations... so maybe leaving it at pain, screaming and regular voice lines (again when idling on foot or interacting with peds, such as attacking or stealing a vehicle from them)