Can I upload a national report which I reference in a paper to a preprint server so it has a permanent location and/or DOI rather than a URL?
It would be very good to upload the reports in a repository, for the reasons you give. You can do it provided this is not explicitly forbidden by a copyright statement on the reports. If the copyright status is absent or unclear, do it anyway, the worst that can happen is that you are asked to remove a report. (This once happened to me after I posted financial data on journal subscriptions on my blog, but these data had never been made public before.)
Asking the authors may not succeed, why would they do it now if they did not do it previously? On the other hand, it would be nice to inform them after you post the reports.