SP could mean special power, or support potential, or anything with S and P for that matter. the point was that there would be items and skills dedicated to making a support heal more shield more buff more etc. that wasn't AP.
SP generally refers to Spell Power (Which in LoL is 'Ability' Power), see any game with stats that include S .. P .. - WoW or Dragon Age are the first to come to mind, and then you also have Yu-Gi-Oh.
That being said, again, AP buffs what you're saying should be buffed by "SP", as well as, gives them some extra resource for damage from non-supportive spells (Unless you include the CC from Lulu's Q). Janna's ulti increases the heal amount by her AP. So does Sona's heals, Lulu's shields, etc.. If there's something I'm missing, then I'm not part of that group that can
meet me half way on this one.