Well, first you'd have to make your extension remove the text option from the StringManagers array.
Then you'd have to write an extension that follows both the rules of html and textile.

I imagine it would be something like this:


// Remove all existing formatters from the StringManipulator object
$Context->StringManipulator->Formatters = array();
$Context->StringManipulator->GlobalFormatters = array();

// Declare your new StringFormatter
$Context->Dictionary["HTMLExtile"] = "HTMLExtile";

class HTMLExtileFormatter extends StringFormatter {
function Parse($String, $Object, $FormatPurpose) {
// Do your string manipulation here...
return $String;
}
}

// Instantiate the formatter and add it to the context object's string manipulator
$HTMLExtileFormatter = $Context->ObjectFactory->NewContextObject($Context, "HTMLExtileFormatter");
$Context->StringManipulator->AddManipulator("HTMLExtile", $HTMLExtileFormatter);



I haven't tested it, of course, but I'm pretty sure that will do it.

have you got something similar to remove the text bullets and allow just BetterBBcode all the time?

About the code Mark posted, it work fine if no comments have been post before with an other FormatType.
With a previous text comment, you get this message:
"Notice: Undefined index: Text in /path/to/library/Framework/Framework.Class.StringManipulator.php on line 98
Fatal error: Call to a member function on a non-object in path/to/library/Framework/Framework.Class.StringManipulator.php on line 100".

To fix it you just need to add in the extension:$Context->Configuration['DEFAULT_FORMAT_TYPE'] = 'HTMLExtile';