The access issue is the most critical problem with grey literature in my view.
I doubt most existing companies would mind if you archive their technical data sheets. Technical reports on the other hand, they probably would mind, particularly given that some companies sell copies of their own technical reports.
For documents from entities which no longer exist, I think one could make an orphan work archive to handle these in general. Your chance of getting sued is fairly low if the entity which owned the document no longer exists and has no successor.
If the report was publicly funded and/or produced by a public entity (even if it no longer exists) then you might have some luck getting the successor or responsible organization to put a copy online. I was able to put this report online because I found a niche archive that apparently has the rights to the reports in this series now. I’ve done similar things for government entities.
For reports which are currently online but may not be online in the future, I’ve used Perma.cc to archive them.
With permission I’ve also uploaded an unpublished Master’s thesis to the Internet Archive. Unfortunately this appears to be poorly indexed by Google; it doesn’t even appear in Google Scholar.
Certain libraries also hold large amounts of older grey literature. I’m a particular fan of the Library of Congress’ Technical Reports and Standards unit. The main problem with libraries and archives for grey literature, however, is that you typically don’t know if they have a particular report until you visit or ask them.
Also related are translations, a fairly unusual form of grey literature. I wrote a long Stack Exchange post about how to find translations. It would be really nice to have a website where you can enter a DOI and find out immediately if there’s a translation. Unfortunately translations went out of style at the end of the Cold War, and as far as I’m aware the largest citation database for translations no longer exists.