MDMService.Preferences is really an awesome feature to provision apps with config data, etc. I think, it's not DIRECT_BOOT aware, though, c.f. here: https://developer.android.com/privacy-and-security/direct-boot, i.e. it will only work after the first unlock by the user.
Is there any plan on adding this feature? Or what would you recommend to consider, when implementing this oneself?