George I would venture to say that most of the Union slave owners came from the border states with Kentucky probably leading the way. There is a Confederate tombstone at the Camp Chase Cemetery however the problem is he was a Union Veteran with a Kentucky unit. His father was able to get his son's United States pension after his death and his father was listed in the 1850 census as being an overseer in Kentucky. Actually there are about seven Union soldiers having Confederate tombstone at Camp Chase.
I'd surmise that the majority of Union slave owners in Kentucky came from the Northern Counties in Kentucky.
Connecticut was the last New England State to do away with slavery in 1848.
The Northwest Ordinance was created in 1787 which in turn became the Northwest Territory and outlawed slavery. The States of Ohio, 1803 Indiana,1816 Illinois, 1818 Michigan 1837 and Wisconsin 1848 were all created out of the Northwest Territory and this is all base upon if I'm remembering my history correctly.
In 1820 we had the Missouri Compromise and again Missouri came as a slave State while Maine was admitted as a free State.
So if I were looking for Union slave owners I'd be looking in the border States of Delaware, Kentucky, Maryland, and Missouri and West Virginia if you would consider it a border State which became a State on June 20, 1863.
There are some minor exceptions like parts of a State such as Louisiana who the Lincoln government recognized around New Orleans in 1863 I think that may have had loyal Union people who may have owned slaves.
Kansas became a State in 1861 and outlawed slavery but in 1850 there may have been some Union slave holders in Kansas but not a lot.
California became a State in 1850 but I don't recall if they had outlawed slavery when they entered the Union but they probably did.
I don't recall seeing a list of Union slave owners prior to the war but that does not mean one does not exist.