For the next release of Koo (5.0.4) on which we're working, we've introduced a custom search feature. It is more or less the same that you'll find in version 6.0 of OpenERP, but we'll make it available for 5.0.

The following video shows the feature, including auto-completion of selection and boolean fields. Currently the query tries to match all filters but we will probably allow users to create OR-type filters: