your idea of a new TO is what I would try. In your parent table, create a global text calculation = "your keyword(s)". Then, create a new relationship (like the one you have now) and include, or exclude, a match between your new global and the child table.
(caveat: not tested; not tonight anyway)
When I need this then I create a calculated version of the ID field (or based whatever field your value list is based ) with something like
Field name: _id_if_active
Calculation: If ( <WhateverCriteriaYouWant> ; ID )
Then base the value list on this new field.
Note: If you have ID and another value that you are using to make the value list and you are only showing the second field then you actually only need to this with the ID field not the second field and your value list will still come up correctly.