What section of Boston (Greater Boston) was home to the bohemians in
the 1920s and 1930s?--the young people who would have been living
somewhat unconventionally and working or dabbling in the arts? 
And--where in Boston might a respectable young woman, single, early
twenties, have been able to live alone without compromising her
reputation?

Thanks for asking!

This has been a fascinating search. Admittedly, I've only located a
single, solid reference to locations of Bohemian artists in Boston,
however, considering the source, it's probably pretty authoritative.
The North End seems to be the neighborhood most likely to be chosen by
the group you describe in the era selected. The Back Bay would also be
a possibility, since ethnic migration from the Back Bay is indicated
in several neighborhood accounts.

"1930s: Robert Chase and other "Bohemian" artists move into the North
End."

Boston Family History
Neighborhoods - North End
http://www.bostonfamilyhistory.com/neighborhoods/neigh_nend.html
Neighborhoods - Back Bay
http://www.bostonfamilyhistory.com/neighborhoods/neigh_bbay.html

You'll find a wealth of Boston neighborhood historical information at
Boston Family History. A time line for each neighborhood details the
historical events and ethnic migrations.

Boston Neighborhood's History
http://www.bostonfamilyhistory.com/neighborhoods/index.html

From this reading, I would conclude there are a variety of
neighborhoods which would be suitable for a young lady of your
description. I believe you'll find the Allston Brighton Historical
Society documents helpful in selecting a more exact locale. Packard's
Corner would seem to meet all your criteria.

Allston Brighton Historical Society 
Neighborhoods
http://www.bahistory.org/bahneigh.html
Packard's Corner
http://www.bahistory.org/PackCornHist.html

Background Information / North End History

Books

Ethnics and Enclaves: Boston's North End. by De Marco, William. Ann
Arbor, Michigan: UMI Research Press, 1981.
The North End. The Boston 200 Neighborhood History Series. Boston:
Boston 200 Corporation, 1976.

Websites

Boston Local Historical Societies
http://www.bostonhistory.org/frameset.php3?t=neighbor.html

Urban photos - North End
http://www.urbanphoto.org/boston/northend/

North End Map
http://www.northendboston.com/tourmap.html

Boston Women's Heritage Trail - "Each of the five trails has a theme:
"A Diversity of Cultures" in the North End, "Writers, Artists, and
Activists" on Beacon Hill, "The Search for Equal Rights" in downtown,
"Women's Action for Economic and Social Justice" in Chinatown and
"Educators, Artists and Social Reformers" in the Back Bay. According
to the guidebook, each takes about 90 minutes to complete. "

Boston Women's Heritage Trail
http://www.freep.com/news/metro/histry10_20020310.htm

Search Terms
Boston neighborhoods history bohemian

I hope you find this information useful. Should you have any
questions, please feel free to ask.

~larre

I think you have given me everything I asked for!  Excellent work. 
Thank you.  I know where the respectable young lady and the bohemian
artist met, and now I have an idea where they went after that.  As a
bonus for me, I used to live in the North End in *my* bohemian days
and can picture it very well!  (I'm far away now.)  But it was
definitely a conservative Italian neighborhood by the time I moved
there and not at all hospitable to the unconventional.  With your
leads, I can now search out how and when it changed.