Unfortunately not (though no my own wish & todo list already).
However (In Lazarus 4.0(RC)), if you want to search within one specific watch, right click that watch, and "Inspect" it. In the inspect Window there is a search field. It searches field names and content. Once found the field of interest, click "Add Watch" and have a new watch for it.
In the watches window, while there is no search, once you have found a field of interest (in an expanded watch), then you can drag and drop that field to a position outside the expanded area. And that creates a new watch too, showing you that field without the need to expand all fields.