ok, maybe this is a svn problem. for git (and i believe for cvs), a tag marks a specific commit and does not make up a commit in itself.
if a seperate commit is the only way then the solution may be in fixing git-svn, or just running a script through all the tags. (for each tag, find the parent commit, tag that commit, remove the branch-tag) git makes such scripting rather easy.
greetings, martin.