Checking the Lightroom 4.4 "metadata" module in the Library tab, and looking at the IPTC and IPTC Extension fields, I don't see those two fields as being available to edit or search.
One workaround might be to upload to Zenfolio from Photo Mechanic and apply a special Stationery Pad during the upload that would hide whatever fields you wanted to omit from the uploads, leaving the fields populated (and searchable) in your local versions.
-Mick