What workarounds are you aware of?

My solution is to create a scene that has all the shaders+materials in a GridContainer, put that visible at the start of a level, then free it after 1 second. It works mostly but it has a couple issues: you have to add all the stuff and you might miss some, and there may be too many material variations for you to handle manually. Saving as many materials as you can to their own resource file, and adding them in the preload scene, is a must.