Are there any discussions or examples of the best way to use Web Parts to
allow users to customise a database web application?

I am thinking of the sort of modifications someone would typically want to
make to a data-driven application:
- additional validation
- change the type of control
- change the text/label
- change the validation message
- add a field
- make fields read-only
- hide fields
- move fields
- add links

I am also looking at how a "designer" can make modifications to an
application and then combine the changes they have made into a new
"configuration" that can be deployed out to a select group of users.