I way afraid I had to rewrite some webpart code but that's the way I eventually solved it. On the bright side that makes webpart configuration easier for users. So thanks alot for your suggestion. By the way, I didn't have to rewrite the LogonForm webpart because I found it if you leave the failure text field empty, Kentico retrieves a resource string for that by default.
Unfortunately the site is live already but I appreciate your help. Thank you.