pseudo code for class that allows you to customise the edit form for any data model

DataObject_CMSEditor.php

<?php

/**
 * Extend this class to create a customised one record editor
 * for your model - within the framework of the 
 * Silverstripe CMS. 
 * 
 * In the model class you identify the preferred CMS Editor Class. 
 * 
 * In the class below, you can set up custom CMS fields, actions (buttons)
 * and the actions that relate to the buttons.
 * 
 * Typically, you would use this class to add actions such as... 
 *  - save and edit next
 *  - save and update third-party-API
 *  - save and publish related page
 * 
 * Think of it like a controller for a model that is solely
 * dedicated to editing the model in the CMS and its associated 
 * actions.
 * 
 * 
 */ 
 
abstract class DataObject_CMSEditor extends Controller {


	private static $allowed_actions = array();


	/**
	 * The model at hand that is being edited.
	 * @var String
	 */ 
	protected $model = null;


	/**
	 * The record at hand that is being edited
	 * @var DataObject
	 */ 
	protected $record = null;
	
	/**
	 * Set the model
	 * @param String $s
	 */ 
	public function setModel($s) {
		$this->model = $s;
	}

	
	/**
	 * Set the model
	 * @param Object $o
	 */ 
	public function setRecord($o) {
		$this->record = $o;
	}
	
	/**
	 * for the CMS fields, we take the fields from the model itself
	 * and we can change them here as and when required.
	 * 
	 * @return FieldList
	 */ 
	public function getCMSFields() {
		//for backward compatability - we add:
		$fields = $this->model->getCMSFields();
		//do stuff
		return $fields;
	}
	
	/**
	 * returns the buttons that apply to the current 
	 * model and record.
	 * 
	 * @return FieldList
	 */ 
	public function getCMSActions(){
		

	}
	
	/**
	 * does something cool
	 * 
	 * @param HTTPRequest
	 * @return ajax response
	 */ 
	public function myaction($request){
		
	}
	


}

MyModel.php

<?php

class MyModel extends DataObject {

	/**
	 * name of the Class that manages the 
	 * one-record editor in the CMS.
	 * 
	 * @var String 
	 */ 
	private static $cms_editor = "DataObject_CMSEditor";

	/**
	 * return the class that manages the one record 
	 * edits in the CMS.
	 * The environment parameter has been added
	 * for future proving.  It is provided to add 
	 * something like: modelAdmin environment, OR read-only environment 
	 * or whatever.
	 * 
	 * @param Mixed $situation
	 * @return String 
	 */ 	
	public function getCMSEditor($environment = null){
		$editor = $this->Config()->get("cms_editor");
		$extendedEditor = $this->extend("UpdateCMSEditor", $editor, $environment);
		if($extendedEditor !== null) {
			$editor = $extendedEditor;
			
		}
		return $editor;
	}

}

The way this has turned out looks like we're completely removing the responsibility of CMS field generation from the model, which is good as it also makes it easier / less bloated to run the framework without the cms module.

IMHO, the models need to nominate the CMS editor, not the other way around. By default we can have a GenericCMSEditor which provides the same functionality as DataObject->getCMSFields, more or less.

Also, getCMSFields on $this->model will surely become redundant as the model delegates that responsibility to this DataObject_CMSEditor - so might as well remove the call.

@willmorgan - agree with everything you say ;-)