Making file immutable in working copy

How do I make a versioned file in a working copy immutable, i.e.,
prevent it from being changed (especially from being deleted) by
subsequent 'svn up' operations?

Removing the write permissions in the file system is not
sufficient. One could use 'chattr +i ...', but that involves
superuser privileges (or special capability settings). There must
be a simpler way.

Rationale: I know that one can always get an old version of the
file from the repository. However, I am concerned with the case
when the server holding the repository is compromised. Yes, then
I play in a backup. However, my users want to be as independent
as possible from any centralized solution; they want to rely on
certain files never changing in their working copies, at least
not without their explicit consent.