although i succedd using bandwidth limiting with cbq-script, ehmm do you heard any bandwidth limiting apps, lately i try bandwidth arbitrator checkout the link site
and some of bandwidth limiting project still ini beta or less than 1.0 version

is it possible using only one interface to limit the bandwidth (i tried but failed - and if i am using two interface i am succedd)

hey have you heard about mikrotik / StarOS - they sell some good router,bandwidtlimiting software (i bet not) have you used on your network environment ? (i need to to know the performance of the software) even this is not free OS, but they give some trial

thanks for your concerned