Help! My Key Lim Pie is too sour!

The meringue may be sweet enough to balance out the tartness. I don't think you need to sweeten the custard.

could you fold in something either sweet or neutral with a similar consistancy? I'm thinking vanilla Greek yogurt, sour cream, etc.

Not much you can do without changing the structure of the custard, but you could try folding in some simple syrup or agave syrup (I think honey would be too flavorful). When you serve it, you could plate it on a sWatch of sweetened raspberry or strawberry coulis (or even some berry jam if you have it in your pantry) to cut the tartness.