I cannot say for sure, but I suspect any web server authentication + LDAP is supported the same way as the "kerberized" (GSSAPI) LDAP authentication is implemented in Observium. You would need to of course use same usernames in the web server auth as in LDAP.
In my opinion from a web app (Observium) point of view it does not matter if the username comes from GSSAPI module or local web server user database. If with "local authentication" you mean Observium MySQL authentication, I suspect the answer is no. But by using web server "basic" HTTP authentication I suspect it would work.
I cannot say for sure, but I suspect any web server authentication + LDAP is supported the same way as the "kerberized" (GSSAPI) LDAP authentication is implemented in Observium. You would need to of course use same usernames in the web server auth as in LDAP.
In my opinion from a web app (Observium) point of view it does not matter if the username comes from GSSAPI module or local web server user database. If with "local authentication" you mean Observium MySQL authentication, I suspect the answer is no. But by using web server "basic" HTTP authentication I suspect it would work.