Can you give us a bit more detail? Something concrete?
Some ideas I've used: facelets tag handlers, dynamically ui:including forms
The idea is that we have a set of products. Each of them has a number of attributes. Each attribute can be marked as searchable which makes them appear on the product search page. Operator can turn the searchable attribute on and off at application runtime. Name of the corresponding to the attribute form control is <product_name>_<attribute_name>.
Currently we are using ajax to build this form on the fly. Application is old, completely home grown, no framework is used.