This is an old thread now but it's become current for me in my work just this past week. I'm situated firmly between the artists and the renderer makers and I write my own plugins for Maya and Houdini to support OSL and 3Delight.
We are wrestling with scenes from a third party that have some Arnold triplanar projections and will be going through the process of outputting the graphs to NSI. I understand the various concerns people have posted and in general I would never let any of my artists create this sort of node graph in our production scenes -- the sort of thing we are running in to upstream from the triplanar nodes would be better served by being baked into texture maps. BUT. This is a very expressive idiom in a shading graph and it seems perfectly logical that the OSL graph should be able to be modeled the same way. This is certainly the way I would think of doing a triplanar projection absent any examples. Namely:
Here's my input.. project it from three directions.
Whether this proposal (and Larry's expression of it above) are the correct one? That's above my pay grade. But I will definitely throw any weight I might have behind the fact that it *must* be solved inside OSL. It's a pretty glaring hole in OSL in my eyes.