I'm aware that SPNEGO is de-facto only used in the wild for Kerberos or NTLM. Is there any research / academic / educational example on how it can be also used for other mechanisms as well?
There isn't really any other way to use it other than how it was designed because then you're not using SPNEGO, but some other protocol you've invented.
What I think you're asking is whether there are other mechanisms used out in the wild other than Kerberos and NTLM, and the answer to that is yes. A handful that I'm aware of are
- NegoEx (an extension to the negotiate protocol that provides more knobs)
- PKU2U (Kerberos derivative)
There are probably more, but these are the ones that are majorly used.