mod_cband (http://cband.linux.pl) can do this from 0.9.6.0 version!
You should use CBandExceededSpeed directive to slow down pages when the bandwidth is exceeded 🙂
See: http://www.howtoforge.com/mod_cband_apache2_bandwidth_quota_throttling
http://www.icewalkers.com/Linux/Software/524590/mod_cband.html
http://www.montanalinux.org/mod_cband.html
http://www.uno-code.com/?q=node/64