What is the nature and history of UL capacitor recognition?
The question appears to be "How do DC Capacitors get UL recognized?"

Or more generally "How does a product get UL Recognized or Listed?"

If the product fall under existing standards (UL and non UL standards) then the manufacturer applies to UL.

If the product does not fit the above standards or a change to the standard is required then the manufacture contacts the relevant standards committee representative (this could be UL ). For example ISO (http://www.iso.ch) or ANSI (http://www.ansi.org).
(The committees are composed of “interested parties” like UL, consumer groups, manufacturer’s, insurance representatives etc. )

Another route is to participate as a member of the UL Standards Technical Panel, I believe it takes substantial time and effort.
But if you want to participate try here ...
http://www.ul.com/global/eng/pages/solutions/standards/developstandards/participation/callformembers/index.jsp

The OP wanted specifically the history of how how the standards were developed for the capacitors.
The work of the committees / panels is recorded in minutes / bulitins however I don't know how to get hold of these.