let me preface my answer with: im a saltwater rookie.
I think that you COULD have a HOB refurium, but would it be big enough to be effective as a filtration mechanism?
If your goal was to just provide a safe haven for tender species, then I say go for it.
If your goal is to provide extra water volume and filtration via algae/chaeto/etc, then I say no, its gonna be too small to be worth it.
I know you said you cant go under the tank, but can you go above the tank ?