RTC Plans: Possibilities to filter for child tree element attributes?

Usecase:
In our backlog we display child tree items (parent/child-relationship or related-relationship) that are filed against categories of some other teams.
We want to filter our backlog items by the filed against-attribute of the child-elements.

I see that I can use the "Exclude / Expression" in plans for "children:" or "related:", but it only allows filtering by the number of children ("set" / "unset" / "<n>" - number of child elements).

But so far I have found no way to filter by an attribute value of the child element (in my case the "filed against"-attribute of the child). Something like "children:!filedAgainst~searchValue" is what I'm looking for.
In queries this is possible, but is ist possible in plans either?

Or do you know a better way of doing this in plans?

Thanks!
Juergen