PROCEDURE TO FLASH eMMC

[Im4Tinker]

So is there a quick way to check whether a given image provides the UMS capable bootloader?

It's difficult. It takes to write u-boot to the media and test it. Practically all the installation should provide such function.
In details it might be possible to scan the first 4 Mbytes in search of a stream of code that is used to start UMS function. Thereafter write a small program that will check such code.
AFAIK, it should need to compile an u-boot without the UMS function and compare it with one which has it. So it will be distinguished the relevant part of code.