+EXTRADLLFLAGS = -Wb,--prefer-native
Yeah, --prefer-native is important here, many apps (and notably CEF pack distributed with apps) ship working icu.dll and that is what gets loaded on Windows too.
FWIW Microsoft's system icu.dll is a fork of open source one with some changes, it is available here: https://github.com/microsoft/icu . And I think forking and shipping the library in 3rd party software is not prohibited by Unicode license. Probably the main problem is that it is C++. But maybe it is possible in theory to ship from a separate fork, technically similar to Wine-Mono or Wine-Gecko? I don't actually see why Wine couldn't download directly from Unicode repo (using an installer similar to Wine-Mono) in principle, although original Unicode Windows dll releases from https://github.com/unicode-org/icu/ differ, most notably, in versioned naming of the dll and exports.