Regarding Supernote...
Quote:
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Users will not be able to sideload in their own apps, but Supernote said that future updates will introduce apps, as people request them.
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Way back in the day, Android phones sold by AT&T didn't support sideloading Android apps.
Using ADB to install apps worked, though.
I don't have one of these in front of me to test this, but there's a non-zero chance you can enable USB debugging and use ADB to do the needful.