I am trying to make
my questions more specific. This U.K. webpage has a section on growing and
training a free-standing fruit tree http://realenglishfruit.co.uk/tree-training/
My understanding from that page is that the resulting tree has a crown that is
not quite flat, but that should be OK for my purpose. The U.K. webpage mentions
rootstocks MM.106 and M.26 which are locally available according to this
Ontario government webpage on apple rootstocks http://www.omafra.gov.on.ca/english/crops/facts/00-007.htm

Considering that I
am after a small ornamental garden feature, what scion shall I be looking for
on an M.26 rootstock?

Why would you need a scion? Just grow the rootstock out if you don't care/want fruit from it. There are ornamental pears, plums, cherrys (male flowers only) that won't make fruit.

Oh, I can buy the
M26 rootstock without scion? I did not know that, thanks! I really like to grow the M26 rootstock and train it to a
circle-on-a-short-stub tree, it will take me about 3 years according to the
U.K. webpage that I linked to.

Edit: I found the tree heights from http://realenglishfruit.co.uk/tree-size/ 8-10' for M26, 7-8' for M9, 5' for M27.

You do not need to limit yourself to fruit trees. A fan is just a form of espalier and many ornamental trees and shrubs can be trained in this manner. Here is a list of various ornamental choices.