"snmp-server enable traps pppoe" doesn't provide that typo of info, nor do I think it should anyway. OTOH, if you configure "aaa accounting" globally or "ppp accounting" per interface, the NAS (your rtr) can report such user info to the AAA (RADIUS or TACACS) backend:
Active Accounted actions on tty0, User (not logged in) Priv 1
Task ID 1, EXEC Accounting record, 00:35:16 Elapsed
Active Accounted actions on tty33, User ellie Priv 1
Task ID 16, EXEC Accounting record, 00:00:17 Elapsed
Active Accounted actions on Interface Async33, User tom Priv 1
Task ID 17, Network Accounting record, 00:00:13 Elapsed
task_id=17 service=ppp protocol=ip addr=10.0.0.1
Then it's a matter of instrumenting a solution on the AAA server to alert/report however you want for the auditors. I'd think SNMP trap wouldn't be the first choice as the delivery mechanism in that case, as there're far more straight-forward options on a server.
Alternatively, If you believe such info can be obtained with some show commands on the router itself, and the router supports EEM, I'd suggest seeking a potential EEM-based solution over in the other Network Management forum (https://supportforums.cisco.com/community/netpro/network-infrastructure/network-management), which can certainly generate SNMP trap accordingly.
