I couldn’t remember if Methley was clingstone or freestone, so did a quick google search. I landed on Arbor Day’s website where I was shocked to see prunus besseyi as the roostock for their dwarf Methley trees. I know they are graft compatible with plums, but I figured they would be too dwarfing to be a practical rootstock for most situations. They claim 8’-10’ high with 10’ spread, but that seems a lot bigger than what I would expect from this combo. I was also thinking it would probably need to be staked or trellised.
My Japanese plums on besseyi are around 7-8’ and I did not stake them. This thread has more details http://www.growingfruit.org/t/compatibility-grafting/4664/23
Somehow I missed this post. What is the spread and growth habit? Does it produce well? One of documents you linked suggests that it produces inferior fruit. I’m not sure what that means.
The thing is Japanese plums flowers always get frozen here. The plants are fine but they are basically one 7’ branch.
On this point Prunus ×cistena would in theory make a good dwarving rootstock since it is a cross between P. cerassifera (generally considered a highly compatible rootstock for many Prunus) and P. besseyi.