I couldn't find a way of monitoring critical servers stats for NetWare servers for trend analysis.... It's kinda crude but does the job for us, and a couple of people who've seen it thought it might be of wider interest.
This tool reads NDS for a complete list of servers in the tree, then gathers statistics for each server and optionally writes them out to log files.
Disclaimer: As with everything else at Cool Solutions, this content is definitely not supported by Novell (so don't even think of calling Support if you try something and it blows up).
It was contributed by a community member and is published "as is." It seems to have worked for at least one person, and might work for you. But please be sure to test, test, test before you do anything drastic with it.