Reference to backlog iteration within a query and portability to process template

I have created a "Backlog" query, to search for all the work items under specific condition; to simplify the description, this condition looks like this:
- they have "planned for" = "Unassigned"  OR
- they have "planned for" = "Backlog"
(idea is that everything that has not been assigned a plan, or it is part of the backlog plan, it is part of my "backlog").

The point is that the query works fine in the project area, but when I try to include this in the predefined queries, I get an error message saying "Planned For references a value that is not part of the process specification.", and even if I go ahead and extract a new process template, the condition "planned for" = "Backlog" is not being inherited in the process template.
Is this a RTC limitation? O something I am doing wrong? How is this expected to be managed?
Thanks,
--Claudia