I am working toward more of a 'code generation' option, which I think presents a variation not mentioned here. In short, you use a specification language to generate the data model and forms - there are some examples of similar things out there, including for the web - ERW
http://erw.dsi.unimi.it/ , written in Java which produces PHP code, and Xdobry, written in TCL which produces forms on an X windows box.
I think there are enough intermediate APIs to make our acs-objects and CR, search, etc available as tcl scripts generated from a schema definition (well, we're getting there).
The extensibility would then arise by adding custom data elements to the schema def, and re-running the generator.
