I agree that it doesn't seem possible to use a schematic filter to select only fitted components of a specific variation.
However, I think I might be able to help you solve your problem.
Go into the Variant Manager (click on the PCB icon next to the variant drop-down menu, or click on Project -> Variants...)
Under the 'Fitted Components' section, you can right click anywhere and choose 'only show varied components' if that's a filter that you want.
You can also click on the 'Fitted' heading under the variant name, and it will sort the components by whether they are fitted or not.
Then you can select multiple components, either by click->drag, or by selecting one and shift selecting another to select all components in between the two, or by ctrl-clicking on each component you want selected.
Once you have them selected, you can modify their parameters in the 'Component Parameters' section.
As far as I can tell, there is no way to filter the schematic based on variant parameters, or parameters modified for a specific variant.
- Erik