I can create a marking guide with a numerical mark, even if the assignment is set to have a non-numerical grading scale.
To reproduce - create an assignment from the new mod_assign, set the grade to Scale: Satisfactory, and the grading method to Marking Guide. Click Save and Display. Create a Marking Guide and set the maximum on a criterion to 100 and fill in all the other required fields then save it.
When changing an assignment where grading is already complete from a numerical scale to Scale: Satisfactory, the grades still exist and are editable using the marking guide.
It would seem that all the places where I have noticed the warnings are missing are due to the fact the testing instructions are not providing a place to actually look for the warnings - they are on the Marking Guide pages, as opposed to the page where the changes are being made on the assignments. Are there hooks in place with in the marking guide that could be called by the assignment module to display warnings when making changes on the assignment page?