The missing MIBs are not so much a problem, there is another bigger one making SNMP very much useless on that device.
I want to monitor my WAG54G (Firmware 1.01.5) with MRTG so I did a SNMP walk on it and was very lucky to find a detailed snmp table.
However the problem is: the Interfaces octects are not counted right.
Following the SNMP device description from the walk interface 2 is the WAN side. The outgoing octects on this interface are fine - but the incoming are not counted properly (raise very slow after some seconds even if there is a continous upstream). I wrote a mail to the german linksys support and now waiting for reply.
You can try it yourself with Getif - it can also plot a graph (useful for testing load on the interfaces).
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www.wtcs.org/snmp4tpc/getif.htm