Thumbcode meets our requirements as follows.
Device independence is achieved by designing Thumbcode around the hand rather than around any particular device, while ensuring that it can be recognized by a variety of devices.
Thumbcode is scalable as a corollary of device independence: as technology permits more compact recognizers, those recognizers can be shrunk without limit. The hand itself of course does not shrink, but by definition no part of our body or normal clothing is an encumbrance, only what must be added to the hand for recognition of Thumbcode.
Thumbcode is digital in two senses. First it provides for the whole of the ASCII character set. Second it consists of a regularly organized system of discrete states. The regularity results from being factored as a product of three bits times four fingers times three phalanges per finger, the five primitive states of Thumbcode, which greatly simplifies recognition. Discreteness is the wide separation of those states, resulting from wide separation within each of the five primitive states, greatly reducing the likelihood of error.
American Sign Language is not digital in either of these senses. It does not provide for the full ASCII character set. And its ad hoc structure means that any recognition strategy will in general need to distinguish states at the level of characters rather than bits. It is much more work for both the user and the recognition method to ensure that every pair of ASL characters is adequately separated than the two, three, or four states in each of the five primitive factors of a Thumbcode state.