Ther is another solution to get this working.
some of my customers have been using a sso application from a company called rsconnect. they have created an em-sso.
the application 'knows' which cucm is online and uses that server to send the login/logout requests to.
you can even setup a list of cucm servers and change the order of the prefered ones. so there is no need for SLB or manually updating IP's anymore