After D8316, we decline to construct queries including UTF-8 outside the basic multilingual plane. It would be slightly better to reject this data sooner, in request handling: if you submit a GET/POST with HTTP data that isn't in the BMP, we can kick you out before doing any sort of processing.
This is mostly nice-to-have and may break a few things (browsers with weird encoding rules?) but is something I plan to pursue after the query BMP stuff settles down.