Not easily.  You can use matplotlib to do it directly (basically, 
duplicate what is done in the .save() method, but save to a 
StringIO.StringIO() object).  I have some patches up at the Sage cell 
server.  If you apply the hunk at 
https://github.com/sagemath/sagecell/blob/master/sage-patches/02-sage-show.patch#L189, 
you should be able to do something like:
sage: import StringIO
sage: f=StringIO.StringIO()
sage: a=plot(x^2)
sage: a.save(f,format='svg')
sage: f.getvalue() # gets the SVG string
Thanks,
Jason