you can try something like

package org.xtext.example.mydsl8

import org.eclipse.xtext.parser.DefaultEcoreElementFactory
import org.eclipse.emf.ecore.EClassifier
import org.xtext.example.mydsl8.myDsl.Greeting
import org.eclipse.emf.ecore.EObject
import org.eclipse.xtext.nodemodel.INode
import org.eclipse.xtext.conversion.ValueConverterException
import org.xtext.example.mydsl8.myDsl.MyDslPackage

class MyDslEcoreElementFactory extends DefaultEcoreElementFactory {
	
	override create(EClassifier classifier) {
		val result = super.create(classifier)
		if (result instanceof Greeting) {
			result.flag = true
		}
		result
	}
	
	override set(EObject object, String feature, Object value, String ruleName, INode node) throws ValueConverterException {
		if (object instanceof Greeting && feature == MyDslPackage.Literals.GREETING__FLAG.name) {
			super.set(object, feature, false, ruleName, node)
		} else {
			super.set(object, feature, value, ruleName, node)
		}
		
	}
	
}

class MyDslRuntimeModule extends AbstractMyDslRuntimeModule {
	
	override bindIAstFactory() {
		MyDslEcoreElementFactory
	}
	
}

or switch to a manually maintained metamodel with default value = true and a grammar like (flag="flag")? with a valueConverter turning "flag" into false

...

Hi Dennis, we have pursued the second approach proposed by Christian. We could share our experiences if you want.

Kind regards,
Niels Brouwers (Altran)