I think your idea of using an adaptor piece is the way to go regardless of what the supplier says.
When I fitted on I made up new lines with a metric fitting on one end, imperial fitting on the other to match the rest of the car. I've made my own lines up before and have all the gear plus a box of fittings so that's my easy option. I suspect that will be the case for the supplier as well, they'll be used to making custom lines and the customer sees the car looking the same as before, whereas with an adaptor they may ask "is that a bodge job ?"
I'd not seen adaptors like the ones you found and suspect that if I had seen one I'd have gone that route instead, even though it's an extra joint (to leak
)
Brian