Within Orion, you can give a try to the below Universal Device Poller:
as well as this report:
There is no need to install a MIB into Orion, you need to find the OID to be polled to retrieve the Serial Number if the above Universal Device Poller does not work.
I am not sure how to do it in CatTools.
try a script command, issue "sho inv" on the router this should capture to 1 file for each device. might cause some extra work but you will have s/n of router plus all cards that have been installed.
In CatTools the Report.Version.Table activity will generate this information and format it into a nice tabular format.
Off the top of my head I can't say whether it will work with Cisco 1801's but we're pretty good with most Cisco devices. Any problems post back here and I'll see if I can help further.
The reports feature as recommended will give you the serial numbers of the Cisco routers. We use this for our Cisco devices along with other vendors, Netscreen, Extreme Networks.