Modifying Available and Chosen Choices For ManyToMany Field With Horizontal Filter In Django Model Admin

Within the Model Admin, is it possible to control what's shown in a ManyToManyField horizontal_filter widget, in both the available and chosen fields?

For instance, I have a County model and a Territory model, and want to hide a County if it's already assigned to a Territory.

The problem with the below is that selected County objects will be hidden in both the "Available" and "Chosen" widget with horizontal_filter. Is it possible to only hide them in the "Available" column?

class TerritoryAdmin(admin.ModelAdmin):
    filter_horizontal = ('counties',)
    ordering = ('territory_name', )

    # remove counties already assigned to a territory
    def formfield_for_manytomany(self, db_field, request, **kwargs):
        if db_field.name == 'counties':
            assigned = Territory.objects.all(
                ).values_list('counties__fips_code', flat=True)
            kwargs["queryset"] = County.objects.exclude(fips_code__in=assigned)

        return super().formfield_for_manytomany(db_field, request, **kwargs)

from Modifying Available and Chosen Choices For ManyToMany Field With Horizontal Filter In Django Model Admin