Piston is a utility that eases vendor branch management. This is similar to svn:externals, except you have a local copy of the files, which you can modify at will. As long as the changes are mergeable, you should have no problems.
Piston has a similar purpose than svnmerge.py which you can find in the contrib/client-side folder of the main Subversion. The main difference is that Piston is designed to work with remote repositories. You should also investigate if SVK would be a better fit for you.
Braid is a simple tool to help track git and svn vendor branches in a git repository. In purpose, it’s similar to piston, but it’s especially built on top of git commands. This allows better integration with git and easier management of merges.