I am making this post because I found a side effect (which is not described in documentation) that some people may find helpful because it allows to setup user account that uses LDAP authentication and still allows the execution of SOAP web services (without LDAP). Furthermore no modifications to OpenClinica-ws are required. I tested this with OC 3.9 and Active Directory as LDAP backend.
Once the LDAP is configured properly for OC instance you can create a standard (local) user and tick the SOAP enabled. The username should match the name of user from LDAP backend. This will lead to the situation where the user is local and hash of the password is stored in OC database, which allows the authentication of user with local password as well as execution of SOAP services with local password. However if e.g. the user uses his LDAP password during the login via web application, OC authentication against local password fails and OC will try to automatically authenticate the user against configured LDAP backend which will succeed (given the correct LDAP password).
This effectively open two ways how to authenticate user using one account.