They are permanently treated as EAPCs by the authorities, though technically they become motor vehicles in law.
However there is already a far superior class of bicycle for the disabled which is why the authorities won't look at your proposal. That is the L1e-A class, pedelec again but allowed up to 1000 watts rating making it more suitable for the disabled with restricted abilities. They need to be registered, but so what, most other disability vehicles need to be too and driving licences are needed.
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