Have you implemented APM Customer Experience Manager (CEM)? This was the recommended approach in the past. There was even a Siebel application type when defining your application details and Siebel examples for user and session identifiers.
There used to be a SARM field pack but I have not been able to locate any details.
You could consider scripting an EPAgent plugin to leverage something like the SARM Query tool output.
Bookshelf v8.1/8.2: About the Siebel ARM Query Tool
Another option might be to use an EPAgent log reader plugin to parse those logs.
Another option might be to use an EPAgent to pull JMX metrics.
Bookshelf v8.1/8.2: About the Siebel Management Framework API Methods