Ok, so things ought to be a little smoother now, as it’s far less likely that any one download can fill my outgoing bandwidth.
I was having lots of trouble getting mod_throttle to work for me, but after reading a tip over at MacOS X Hints, I’ve added mod_bandwidth and got it working without any trouble. So far, only Nicci’s music site has any restrictions, with files under 18KB having no limits, and everything else being capped at around 8KB/s.