@tschopp
Unless someone is aware otherwise, we believe it is expected functionality by design and would not be able to be modified, except maybe in some very complicated way that modifies the Save functionality in Model Driven App.
Save button from Model Driven App only saves info from Model Driven Form and update the Context in Canvas App. It would not really make sense if it did something else.
So out of the box it would not be supported to do it like that.
Instead you should do like this like you said yourself:
@tschopp wrote:
If I put a Save button on the canvas app, I can a issue a SubmitForm() that saves the model-driven app form data, but not the other way around. I would prefer to not have a Save button the canvas app.
That is the way you should do it in our opinion.
Using some potentially unsupported or non-recommend methods, it may be possible to achieve your goal as you state it exactly, let us know if you would really like us to check it for you, and if we have the extra time we may check it, but even if we provide you with such way it may break any time in a future update and it will use the unsupported and non-recommended ways only (we have to check if it is even possible at all, even like that).
However for the clean official answer we believe it is unsupported at all to do what you are asking and the part we quoted from your own words above would be what we recommend is the way you should do it.
Just in case we also welcome the alternative perspective from someone else in case we were actually not aware about maybe some clean way to do it, if someone else knows how to do it please feel free to share it, we'd be interested as well out of curiosity.