Well, I could be totally off-base here, but... I'm pretty sure that a load on demand addon has to be "called" in some way. If you set atlas or superinspect to load on demand in their TOC files, the game would be capable of loading them up when needed, but there would be nothing to tell them to load when you're actually in the game. All the menus, buttons, hooks, or whatever that normally would cause either addon to appear wouldn't be there since they're all part of what you're making load on demand (and therefore they wouldn't get loaded.)
You would have to have some part of an addon that isn't load on demand and is always loaded, so it could call up the load on demand bits.
In short, I'm pretty sure this would require some re-writing of the addons in question, or at least some custom made addons to fulfill this function.