I'm sure that registered KISS users can still buy spares, so get your KISS shipped over sans valve and get a friendly owner to 'lose' their recently acquired spare O2 valve....
But seriously folks - you do need some form of
education in the use of an RB. Short-cutting the formal training route may work for those with the right balance of inquisitive nature, technical competence and self preservation instinct but if you're going to appeal to a wider market then you need to provide some level of instruction and (more importantly) assessment.
Knowing the KISS instructors for the UK I'm pretty sure that if someone with the right experience spoke to them, they would provide a course tailored to an appropriate level without a nanny-state requirement to have all the right stickers in log books.