Hi Bill,
The first Loyalist settlers in Norfolk, (according to the Ontario Historical Society, c. 1900), arrived in 1793. The name Marr does not appear in the list they provide. (reproduced here). The name Marr also does not appear on the UELAC Loyalist directory, although being omitted from the list does not rule out being a Loyalist, and I can also see a possibility for confusion with old handwriting or phonetic spellings where the name could be rendered as Mann, Marks, Maher, or other names (link). I can also find no Loyalist Claims under the surname Marr on Ancestry.
Of the 62 Marr's that are enumerated in Woodhouse Township, Norfolk Co., in the 1851 Census of Canada, the eldest 2 Marr's living at that time, Lawrence Marr, 78 and David Marr, 61, were both born in the United States which means a Loyalist connection is possible, but simply being being born in what became the United States and moving to Canada near the time of the Revolution, does not a Loyalist make.
Perhaps most telling is the fact that in the "Pioneer Sketches of Long Point Settlement" the sketch on the Marrs of Norfolk does not make any claim about them being Loyalist settlers. It gives the familiy's first arrival as Lawrence, aged 27 in 1800. It also says he he was accompanied by his wife and younger Brother David who was "not yet in his teens" and that they preceded Lawrence's father in arriving . Sounds like Lawrence and David described here are a perfect fit for the two individuals I mentioned i the preceding paragraph :)
This same book does give me a very clear idea where the family claim to being Loyalists originates. The book claims (unfortunately without sources) that Lawrence Marr's wife, Rachel, was the daughter of a Colonel Butler of the British army. Given the name, rank, and the proximity to the Niagara area, I might speculate they mean Colonel John Butler, (in)famously the leader of Butler's Rangers during the American Revolutionary war - but this could well be an erroneous assumption on my part - I'm not sure how many Colonel Butlers served in the British Army at that time. The major strike against this being the well-known Colonel Butler (who is known to have had a surviving daughter) is the statement "who lost his life in the conquest of Canada". The Colonel Butler of Butler's Rangers died in Niagara-on-the-Lake in 1796 of age and infirmity. Of course, the suspect statement could simply be the authors embellishment.
If Rachel was in fact the daughter of any Loyalist, she and all her Marr descendants to the modern day would also be United Empire Loyalists.
You can read more of this book I have discussed above here: http://www.ourroots.ca/e/page.aspx?id=182019
Also, whether they would hold any value for you or not I can't say, but you may be interested in knowing that there are Census records for Woodhouse Township for the years 1812 and 1829 (Library and Archives Canada Microfilm: H-1100).