.svn directory - safe to add files in ".svn" or ".svn/tmp"?

[jrm]

Working on some tools for a development environment that will make use 
of SVN.   There are circumstances where I want to create temporary 
information of my own, related to checked out versions. Rather than 
cluttering up the working directory with other special directories and 
files, I was wondering how safe it might be to make direct use of the 
".svn" directory for my own purposes?

Aside from the existing purposed subdirectories - would it be safe to 
create files in either "tmp", ".svn" proper, or a new subdirectory under 
".svn"?

Thanks!

-jrm