Magento. Change layout on fly in depends from current customer group.
If you need find a products what not applied to website you can execute this query and it return a list of all product ids whta not applied to any website.
Can also improve this query for retrieve any need fileds besides entity_id like a sku etc, or can set specific website id for check it.
Initialize event in front section
Add observer for catch event
Setup in our layout blocks with custom markdown for csutomer gropus. We can use this for any conditions.