In order to filter for QPack items by specific values of custom fields simply use the search field 'Custom Fields' in the advanced filter.
Since custom fields are managed separately per each project and per each item type, in order to filter by
their values you first need to specify the project name and the item type in the filter interface.
(For instance: QPack allows Defects in project to have custom fields that are different than the custom fields of Defects in other projects)
Once a specific project name and specific item type are selected, the search field 'Custom Fields' will be
filled with the custom fields names of the selected item type.
This logic also applies on filter grids: when you filter for items from various projects, or filter for various
item types from the same project custom fields columns will not be visible on grid.