'Fraid not. When we go that far back, I have much the same info you do. I was going to refer you to a McKay family lineage posted on the islandregister.com site until I noticed that you had posted it. In fact, I altered by own gen database to reflect the lead para of your posting, as I took a good look at some of the ages and connections I had, and decided that your info was better than mine. Given my conversations w/ Paddy G-B (skydragon), we're all running into the same brick wall.
I have a 1938 letter from Florence Mabel MacKendrick (then living in Ch'town, PEI) to an unknown Mr. MacKendrick (where??), giving some family history, but it doesn't have much we don't already know. Paddy G-B was quite interested in it as Florence is her husband's grandmother. Also have a brief 'History of the MacKendrick Family' written by William's son Allan (1844 - 1921). I think it's been pretty widely passed around in other forms, as I've seen the same info elsewhere. I can forward electronic copies of both these to you if you like.
And no, I'm not in touch w/ any of the family who moved to Taunton, MA. Worst yet, I lived in New Hampshire many years ago and was thru Taunton, without knowing any of the family had moved there.
Other than that, I'm as stuck as anyone else. I'd be interested in comparing notes with you on the McKendrick and McKay families anyway, as I'm sure you have lots I don't and vice-versa, altho it will be more recent than what you're after.
Holler at me directly if you want copies of the files.