The rural-urban model would likewise require defining new electoral boundaries. The rural-urban model does have potential, because the number of top-up seats is relatively limited, about 15% or 12%, if you use more multi-member ridings. You could, in fact, do essentially an STV grouping process and then define regions covering some adjacent STV districts with additional top-up MPs.
I think those are the ways in which that might work out.
In terms of the constitutional issue, I am not a constitutional scholar, so I don't know, but if the numbers are all rising by 12% and you keep that constant across the country, I don't see that anybody would have cause to complain.