I would dearly love to find this, but I am coming up with nothing.

I tried searching in Google Books for various phrases such as "room for one more" changing the spelling to "room for ain mair" and various variations; also for "Lachie MacLachlan" and variants such as "MacLaughlan", "MacLoughlin", etc.

Do you suppose Sorche Nic Leodhas was pulling our leg, and he made up the whole thing himself?