OK, I understand what the inverse function is from your description, but I don't really see how you could do that from a general waveform. You've got a waveform y, and you could use the xval() function to get hold of the x-values that this came from.
I'm struggling to see how you would know how to get the inverse function in general if you'd done some intermediate function, or why you'd even want to do this? It sounds more like something that would be useful if you were doing symbolic analysis, rather than looking at simulation results.
Even matlab doesn't have a generic inverse function - yes, it has the inverse of a number of operations, and things like matrix inversion, but I'm not sure what a generic inverse function would do. Maybe if I knew what you were actually trying to do with such a function, I might be able to suggest an alternative strategy?