This may or may not be a long shot in your quest for information regarding
working class Welsh interiors, but:

The Empire Mine, in Grass Valley, California was worked largely by Welsh
miners brought over from Wales because of their long-standing expertise in
deep shaft mining. As The Empire Mine was in operation from the 1850s all
the way through to the 1950s, chances are they may have some information
regarding the lives of the people who worked in their mine and lived in
Grass Valley.  You can contact The Empire Mine at:
Empire Mine State Historic Park
10791 E. Empire
Grass Valley, California
Phone: (916) 273-8522

There is also a walking tour of Grass Valley. Their phone number is (916)
273-4667. Perhaps the folks who organize this tour might be of assistance
as well.

Another idea is the Nevada City Historical Society. Nevada City is close to
Grass Valley, it's also one of the most picturesque towns in all of
California (my opinion!). I don't have an address handy for them though.

Sorry not to have more specific information regarding the above mentioned
sources. I took a quick spin of a web site called the Sierra Highways
Tourbook. Their address is:
http://www.psnw.com/~sierrahigh/NTourbook/Tg_i.htm

There may be more pertinent web sites out there for you to access. Perhaps
the California State Parks have a web site. If so, they would be a good
resource for you too.  If I come across anything I'll forward the
information on.

Good luck!

Paula Freedman
San Francisco