Getting all that functionality with a plugin will be difficult. The xml form system Joomla uses for its forms/tabs is pretty complex in how it builds itself. Also to my knowledge there is no key "events" at stages that will let you change what form XML may be loaded (that and a Joomla update that renames a couple things or adds another field would ruin it).
But here is my thought.
//regex
$regex = '/\<input[^>]*\stype=\"hidden\"[^>]*\sname=\"jform\[params\]\[plugin\]\"[^>]*\sid=\"jform_params_plugin\"[^>]*\svalue=\"1\"[^>]*\s\/\>/i';
//check for match
if(preg_match($regex,$buffer)){
//bind an xml form
$xml = simplexml_load_file(PATH_TO_XML_FILE.'.xml');
if($form = JForm::getInstance((string)$xml->name,$xml->asXML())){
$arr = array();
//here you need to build an array to auto fill the data from the form,
//if you already had this working to an extent I am sure it will be fine.
$form->bind($arr);
}
$content = '';
//here you need to basically get the form fields into content as text to be added to
//the current output, there are many ways to do this.
//replace the current output of Joomla to add the extra fields
$buffer = preg_replace($regex,$content,$buffer);
JFactory::getDocument()->setBuffer($buffer, 'component');
}
To explain, first the regular expression is finding a form field in the current output of the page (this can be ones from your options tab). If it finds a match it loads an xml file of your fields and then replaces them with yours. Given that you had said that you got some distance I am sure this might only build on that if anything.
