I think the language could be made expressive enough, and the developers thought so too when I asked them about it. I think they're focusing on other things for now though.
A Hough transform is kind of a weird beast; not really your typical image processing problem. For anything that looks like a series of convolutions or other simple image operations, Halide is going to be the fastest thing around; even faster than a series of calls to the Intel IPP library (because of the ability to tune memory access patterns across multiple stages at once).
Can't claim to know what a Hough transform is, but do you think the lack of expressiveness can be fixed without interfering with the tricks that make it work? It's still a language under development after all.