It is still impossible to search/filter for any information entered in the IPTC Extension metadata fields like Person Shown, Release Status etc. They are also still unsupported in the metadata painter tool. I really hope that changes in the final release, those things should have been there when those metadata fields were introduced in Lightroom 3.
The Any Filter plugin lets you search IPTC Extension fields (as well as many others not supported by Smart Collections).
North America
Europe, Middle East and Africa
Asia Pacific