Domino (at least pre 8.5) doesn't let you use $$SearchTemplate forms as anything other than vanilla Domino-generated html forms.
This makes it somewhat difficult to search/getting Json-data in return.
The simplest workaround I've found is to make a view with one column. The data is formatted as Json, with a trailing comma. In the search-template, I wrap the $$ViewBody-field like this:
I'm terrible at explaining code (terrible at explaining anything), so I've made a simple demo-app you can try. Remember to create a full-text index on the demo-db before trying it out.
Flash demonstration of the app (I parse the Json to HTML):
The app is tested, and works in Opera 9.5, FF3 and IE7 on Vista x64.