It is technically possible. There is a plugin from before +relativenumber was a feature, called RltvNmbr[1]. It's a bit broken (doesn't update the line numbers very smoothly) but it displays the relative numbering side-by-side with the absolute numbering.

I just don't use relative numbering enough to warrant trying to re-write it. Maybe you do?

[1] http://www.vim.org/scripts/script.php?script_id=2351