There are two ways to achieve this.
1. You can use wget, it does work. You need to pass the correct parameters, and if you have an authenticated system then the authentication as well. If you are using routers2 internal authentication, though, this will not work.
2. You can call routers2 from the command line on the MRTG server, it has special command line options to allow you to pass the device -D, target -T, style -s and type -t. You can also pass the authenticated username this way if you have authenication enabled. Older versions have a bug that still outputs the HTML header in this case, so watch out for this one.
Authentication can mess things up as routers2 will not hand over the csv if it thinks you're not entitled to it.
[root@seta etc]# /u01/www/cgi-bin/routers2.cgi -D hosts-mail/groucho.cfg -T groucho-load -C -U sshi052
"Hostname","Target","Sample Date YMD","Sample Time HHMM","Count in seconds","Load average","Peak load average"