No clue on Audi, but when I needed one on my 205, I ran a 309 rear washer bottle.
I know some older Toyota's have rear washer bottles too.
Maybe something to look into.
Failing that, a washer bottle can be anything. I've seen then made from cheapo alloy drinking bottles etc. Just need to drill the hole for pump.
This can the be mounted anywhere you like. Not ideal, but beats no washer bottle.
Re MOT, this will be a grey area I think. As we can't prove they are fitted. What if you retrofitted an S3 bumper to a non S3?
Same way we can't test self levelling setups.
It basically says if fitted, should work. But you may not be able to test etc.
So it's a loophole for after market kits, as wording is 'if fitted' not 'must be fitted'.
Will come down to the tester really.