You can use the parameter BASE_LOG_DIR to point to a different directory,
The PPM application (ppm operating system user) will also need to have write access to the directory where you will store the log files. If this option gives any issues, then other option is to create a script on the O/S (by the o/s admins), and schedule it to run every say 15 days, and then move the log files every 15 days old (or so), into another folder.
We handle this in a similar way to what Darshan describes. It is a bit complicated, but it works pretty well.
Basically we have a crontab that runs twice a week. It looks for requests that are between 30 and 15 days old, and archives their execution logs to a zip file.
One thing to note is that on Solaris, you are limited to subdirectories in one directory. This is hardcoded in the kernel. If you have more requests than that at any given time you'll need to come up with a solution in addition to the one Darshan and I are suggesting. We are currently looking at creating a subdirectory structure to do just that.