I like how (normal) git won't let me push if I haven't rebased with the latest code. However with git svn, when I do git svn dcommit it automatically rebases (i.e. git svn rebase) and then commits. Is there a way I can have it throw up an error message if there is new stuff to rebase?
No way. There is always a chance that somebody will commit right after you made any check-as-you-are-looking-for and right before you actually commit something. Just run git svn fetch every time before git svn dcommit to be almost sure git won't rebase automatically.