Like Frank, I have spent a fair while thinking about this (I'm not a Machrihanish member, but it's my favourite course anywhere), and I share his view that the most practical solution is to combine the last two holes into a par five, and make a new hole out in the dunes.
I am not desperately concerned about balance, but one thing that does strike me is that if the new par three came before the existing ninth in the routing, you would have a front nine of around 3100 yards, par 34, and a back nine of close to 3600, par 36, with three threes, three fours and three fives, while if it fell after the ninth, the back nine would have only two par fours!
It's remarkable, at first sight, how many links have either fairly plain (I won't say dull) opening or finishing holes, or both. I thought about this a while too, and in the end it is not so surprising - roads and settlements tend to be on the flatter land, not the dunes, and thus the course had to start and finish where there was access.