I'm confused. So, skinned lists already implement a padding feature (it may be incomplete or buggy, but we'll get to that). The proposal is to introduce equivalent/similar functionality into the core. Rockbox will then have two implementations, one that people can choose to use when skinning, as well as a separate setting that 'just works' even if you don't use themes.
Realistically, how many users use rockbox without a theme? Could the theme functionality be improved/expanded to address the concerns with the existing implementation? If not (if people really use rockbox without themes, but find the need this specific functionality) could the implementations not be unified in some way? Does rockbox really need two implementations of (as I understand it) the same functionality? Clearly I must be missing/misunderstanding something, but not clear what.
