reconciling/registering local customizations with dpkg/apt?

[Krzys Majewski <majewski@cs.ubc.ca>]

Anyone  have a  good way  to  register local  customizations with  the
Debian package mechanism?  For example, suppose file X  is provided by
Debian  package Y.deb,  and I  patch X  to provide  some functionality
local  to  my  domain. I  would  now  like  to  protect X  from  being
completely   overwritten  by  subsequent,   possibly  non-interactive,
updates of Y.deb. I could of course build my own Y-local.deb, but then
I   wouldn't   be  able   to   benefit   from   updates  to   Debian's
Y.deb. (Moreover, I  would then need some way  to resolve the conflict
of ownership of X between Y and Y-local.) 
-chris