Hi,

(Roughly) what proportion of composition students are able to do
something related to music after their studies?
What, more specifically, do those that *are* able to pursue musical
careers end up doing?

The information you're asking for is very hard to pin down. Here are
some starting points for the research.

http://www.menc.org/industry/job/careers/careers.html
The National Association of Music Education

http://www.menc.org/industry/job/career.html
A Career Guide to Music Education

http://www.menc.org/industry/job/careers/careers.html
Careers in Music

http://newtimes.rway.com/2004/upbeat04/upbeat1.shtml
Musical Chairs 
Those in the know dish about the value of studying music in college 

Another option for university students is a music composition degree,
which requires proficiency in orchestration and arranging, as well as
writing for both instruments and voice. Not surprisingly, this is an
uncommon choice of major, and the music composition program at SU
usually only enrolls five or six students at a time. Graduates with
bachelor's degrees in composition usually pursue further education,
often leading to doctorates and teaching positions at universities.