Hi Robin,
Setting variable values on datasource level is not possible now because all the variables are currently merged by default. Design Studio is not yet supporting unmerge variable case (which is already supported in Analysis Office).
When Unmerge variable option/switch is available in Design Studio, your usecase should be addressed.
As far as I know, this is not planned for 1.3. I would advice you to add this to ideas place as soon as it is open - https://ideas.sap.com/ct/ct_list.bix?c=6055F3C4-E8DA-454F-9C49-85D5A5070BCE
Regards,
Bharath