You can script for FMP to do the search on every layout,yes. But, how are you planning on seeing the search results. You'd have to have a layout with related portals or fields to show all results.
Describing your database in more detail and explaining why such a capability would be useful to you could be very helpful. Keep in mind that the layouts in a Filemaker database can all refer to the same table, each to a different table, or some mix of the two. This aspect of your database design will be a key factor in how to set up such a "search everywhere" tool.