This truth just hit me with the force of a hammer blow to the forebrain:
The Update action in the Webpublisher XML API mimics the Inmagic import function. It is not an update by query.
O merciful god in heaven. Import matches on one or more fields which have to be unique (in combination), which generally means a record id. If you don't know in advance which records might be affected you have to do a query to examine the results then perform the update. Two HTTP requests.
What I really want is the equivalent of a WHERE clause: update field X for every record found by the query.
UPDATE: Wait a minute. There *is* a batch modify option showing in the Webpublisher schema documentation, but not in the online help. The investigation continues...