Question ExposedReference for basic types?

Is there a way to create fields on custom Timeline clips that can be overridden per Playable Director? Essentially, I'm looking for something similar to how ExposedReference works, but for types that don't derive from UnityEngine.Object.

I'm guessing this isn't currently possible, as IExposedPropertyTable only has functions that return UnityEngine.Object. If anyone has found a way to make this work, I'd love to hear about it, though!