Thanks for the hints; I've put it on [1] complete with upstream and pristine-tar.
The new build (with 5.2.0) is somewhat simpler and easier to maintain than the previous one, and it's almost working now. I've got a bunch of PRs lined up upstream [2], but if they don't land quickly, I'll include the corresponding patches. Also, if we could convince the maintainers to remove the nonfree part of the code [3], the scripts would get even simpler.
Cheers,
Nico
[1] alioth:/git/debian-science/packages/superlu.git