I think I've just identified it!! I've posted part numbers in the 'Components used on the Switch' thread (can't post link as a new member)
I've just bought some, so will test them out and see how it goes.
Hours of searching through datasheets later, I think I've identified them. They're all transistors (SOT-723 package).
"42" marking = DTC043ZMT2L
"XK" marking = RN1110MFV
There are some others on the joy-con boards marked "49" and "69".
"49" marking = DTC043TMT2L
"69" marking = DTC024XMT2L...