With git-svn, my self-signed certificate can't be validated. Can I skip validation?

With git-svn, we don't have any chance to accept the certificate in Tower because git-svn doesn't provide any parameters for this. You will have to accept the certificate permanently (by pressing "p") when cloning via the command line.
After having cloned the repo on the command line you can perfectly go on working with it in Tower - simply by adding it to Tower via "Add Existing Repo" like any other existing local repository.