The ECMA-402 Intl.ListFormat proposal  has a formatToParts method. To implement this in ICU, we'll need a method in ListFormatter which takes a FieldPositionIterator. Ideally, we'd have this in C, C++ and Java. The first step will be a design proposal, which I plan to write in the coming months.
reopen -> accepted after sffc rewind the master after the merge commit of PR#106