I have left a comment on http://b2evolution.net/news/2014/07/03/state-of-the-evolution-july about this:

All migration tools we tried created so many side effects that we decided against them. Migrating code repositories would have been a (meta) project in itself which we didn’t have time for.

There has been a CVS mirror since a few years already: https://github.com/whissip/whissip/tree/cvs-mirror

It got created and keeps being updated using the following command basically (remove the spaces around the at sign):

git cvsimport -d :pserver:anonymous@evocms.cvs.sourceforge.net:/cvsroot/evocms -p -A -r import -k b2evolution

Then you could apply your patch/diff from SVN<=>CVS on top of it (or sync the current SVN state to CVS before importing it).

This would not include the history from your internal SVN repo then, but at least it would preserve the public history of the project.