The main problem with using git and svn is that git will rewrite its history when pushing the changes to svn.
A one way sync from svn to git would be an option. Git users can provide patches easy enough if they would want to.
I'd prefer the ispconfig organization to setup a github organization but I could easily set up the svn sync in a cronjob on my server. Or you can do that yourselves too
I'm vStone on github, feel free to poke me for any git relates issues.