looking for something like a union file system

[Aryeh M. Friedman]

I am using a dev tool that maintains a "split" source tree for currently
worked on files and those in the repo (aegis which is slightly different
then how svn or cvs does it) and my the default build system assumes it
is all in one tree.... thus I want someway of merge the two dirs and
have a copy on write via a special command for it (i.e. if I start to
edit foo.c it automatically checks it out for me).... any ideas?