Functionally it would work, but I rather doubt it is a good solution from the non-functional perspective. Mika's solution was an SQL snippet, why have you drop this approach? I would create a select to deliver a list of strings rather load all tasks with all search attributes and reduce inside the application.

In addition, I believe going over the payload in JPA might be a wrong way. We have special keys making sure that we create a path-like structure (attr1.attr2.attr3) and exclude some others. The results ends up in the element collection - and those distinct "keys" and "values" are of interest.