Without more info about your hosting situation i am unsure if this would work, but here is a stack that will try to force redirect-canonicalize all host/sub/domain/ip instances into http://www.addon-domain.com
:
RewriteCond %{HTTP_HOST} ^123\.123\.123\.123 [OR]
RewriteCond %{HTTP_HOST} ^www\.123\.123\.123\.123 [OR]
RewriteCond %{HTTP_HOST} ^d5151150\.host-domain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.d5151150\.host-domain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^addon-domain\.my-domain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.addon-domain\.my-domain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^addon-domain\.com$
RewriteRule ^(.*)$ "http://www\.addon-domain\.com/$1" [R=301,L]
If you encounter the platform install path showing once the domain is redirect-canonicalized, then this is a different issue -- you should use a rewrite for that or make the addon (and util sub) "live" in the install folder of said platform.