I was wondering if there are already some thought on exporting linking information for packages providing a virtual dependency.

In the case of blas/lapack the way to link with a given package could change. For now only netlib and openblas package are providing blas. Openblas already has a trick to have a libblas.a

But if at a point atlas also provides blas it is already not so easy to link with it. Especially if you consider the shared libraries that can be build as a sequential or threaded library libsatlas.so and libtatlas.so. This becomes even more complicated if you want to link with libraries like the mkl.

For package using cmake as a make system a way would be to set the environment variable BLA_VENDOR. But I guess that this would cover only few packages.

For all the others, a solution would perhaps be to have a way for packages to express in there spec different {c, fc, cxx}flags and ldflags that can be used to link with them.
With variation of this flags with a default that should always be there and then variation of them for example for threaded atlas or mkl.