Can I throttle tar without ionice?
You can use ratepipe.
I downloaded and tried it out and it seems to DTRT. It should work for you:
tar --remove-files -cvif - 2011-08-02_14-05-09 | ratepipe -r 20 > 2011-08-02_14-05-09.tar
You could probably write the tar to stdout (using -f -) and pipe it into something that throttles. I'll see if I can find something quickly.
Since tar will not buffer much, it should wait on the reads.
Edit: MikeB beat me to it: ratepipe will do.