One thing you can try is getting the facets with and without the filter within the same query.
Check Multi-Select_Faceting_and_LocalParams
- Return the Same facet with and without exclusion
- If No filters they would be same.
- If filter applied, the Normal facet will have filtered Facets and exclusion facet will have facets without the filter.
fq={!tag=dt}doctype:pdf&facet=on&facet.field={!ex=dt}doctype&facet.field=doctype
- Compare at Client side for the differences and display accordingly.