Custom attribute editor

One thing we see at a lot of project we do for clients, is the wish to have a custom attribute editor in the SL and/or HTML viewers. Now it is just possible to show or hide a field, but we would like to do this conditionally. For instance: field A should be (in)visible if field B has a specific value. Or, field A should be disabled if certain conditions are (not) met.

Another 'nice to have' would be an option to set default values, e.g. the current date/time for a date field, current username, etc.

It would be great if this could be implemented in both the SL and HTML viewer, but if I could choose, the HTML viewer would have the highest priority.