There would be no problem integrating it, since it has a C++ wrapper. We already have Rust running, and this would be built at haikuports as an external package (as ICU already is). So, yes, if this is actually smaller and faster, and already implements the features we need, let’s do that.
It will, however, mean that Haiku itself is running on ICU4X, but all 3rd-party code (like WebKit), will be still requiring ICU4C, since the two projects are not API compatible. And so we end up shipping both ICU4C and ICU4X in Haiku releases, and in the end the overall total is larger and uses more memory. Oops.