I would expect it to be not too difficult to get Nagios to query an snmp device and alert based on it. Surely it's a fairly common requirement to alert based on traffic for a switch/router/etc. (although it's slightly tricky with network throughput as you really need to look at the differences between readings to know what the throughput is). I only use Nagios for basic service (http/ping/smtp/etc) monitoring.
As SirDice says you can also just write a simple script that pulls the info from FreeBSD and outputs the relevant string and exit code depending on the result.
Yeah, that's what I would do. But I like to use SNMP solely for network devices that have no other way to get statistics. But, as you say, it can be enabled of course and would provide a fairly 'standard' way of monitoring bandwidth usage.