My company has deployed SER on a very large scale across the US and I would like to develop a way to monitor critical parts of SER and the servers that SER runs on.

I have several questions regarding monitoring SER.

1. Can SER be SNMP enabled via something like SMUX or AgentX from net-snmp?

Yes, a module can easily be implemented.

2. Does SER have any native SNMP support or other monitoring/logging hooks

built in?

There exists an old SNMP module that has not been maintained for a long time. It will most likely not work with today's code, but it can be used as a starting point. We would gladly accept a contributed SNMP module :-)

3. Do any mibs exist for SER / SIP?
4. I am also still trying to find out if SER has anyway to detect when

a user looses registration either via SER internals or external watchdog scripts etc.

I'm not sure what you mean by "loosing registration". By definition, the user is registered until expiry, but it may well "loose" registration if it's behind a NAT that closes the port. However, I assume you mean that the expiry is triggered without having received a new REGISTER. Look at the register module. You will find code that cleans up expired contacts. I'm not sure if there is an existing hook for other modules to use. g-)