Ok, here's what I managed to find so far:
I can display the 'FindOnPage' bar by uncommenting some code in the App Menu. I can enter text (even though it's hidden under the keyboard). When I press 'Enter', the first instance of the word is found and highlighted. 'Prev' and 'Next' buttons don't do anything.
Relevant files are in /usr/palm/applications/com.palm.app.browser/source/