Long story cut short - very old building, second hand kitchen, new
dishwasher.
Without going into details, there are two good reasons for not being
able to run the recommended 32mm standpipe behind the cupboards and
through the 2'6" thick outer stone wall, in which a 23mm hole could
easily be made, but a 32mm one would be "a nightmare" (plus I'd have
to hack holes out of the back of the kitchen units.

One builder and one plumber can't see any way round it other than to
just have a 23mm wastepipe.

As long as there was a good fall (there would be - 600mm over 6 feet -
mixing my measurements here!) and the top of the pipe was the 800mm
the manual specified, can anyone see anything wrong with it?

The entire pipe run will be sealed, so no trap needed inside.

I guess what I'm trying to get at is - it's not ideal, but in the case
of their really being no other option, is there any reason it won't
work? And I've already thought about build up, but as the wash is
about 90 degress and full of evil chemicals anyway, I can't see it
getting the kind of fatty deposit buildup associated with cooler sink
water and normal "hand strength" detergent.

Just to pre-empt questions about building and water regs, they're not
going to actually fit it themselves, it was left as "regulations, see,
but if you thought about it, you could...", and the outflow WILL be
teed into the sink waste, just outside instead.