If he were to get this implemented I think it would be totally withing the perview of every other denomination to say they would not work with any UMC member in any ecumenical context.
I think so too. But I'm afraid that several other mainline denominations would side with the UMC if it came to that. The UMC as a whole probably isn't much, if any, farther down the skids to apostacy than many of the other liberal mainline denominations.
In the late 1960s I left the Southern Baptist Convention church where I had been saved and a member for years and joined an independent Baptist church because of the SBC's movement in the same liberal direction that the mainline denominations were already taking. But after conservative, bible-believing Baptist leaders like Adrian Rogers, Paine Patterson, and W.A Criswell managed to stop the SBC's drift into liberalism, remove liberal, modernist, and agnostic professors from SBC seminaries and some SBC supported colleges, and basically return the SBC to it's fundamentalist roots, I wouldn't be reluctant to join a SBC church again if it was practical for me to do so.