I would like to use a psychology framework that has been published by a third party but amend one aspect of it. I intend, of course, to reference and attribute the model and make it clear that the amendment is mine. Would that approach be in line with etiquette?
Etiquette is neither here nor there in this matter, as pretty much all of academia is based on taking what has gone before, and augmenting / amending / modifying / building on it.
What matters is whether what you are doing is rigorous. Referencing, attribution, and identifying your own contribution, makes the writing rigorous. As to the crucial question of whether or not your amendment to the framework is scientifically rigorous, is only something that someone close to the subject can know.