Hello Sandro,
We are in the process of adding override support for WLANs for E-series and R-series devices. Part of this will be to allow per-device overrides for Enabling/Disabling individual WLANs on E-series devices. This should make it possible to create a single AP Group with both BASE and CLIENT Mesh WLANs and then enable/disable the proper WLANs on a per-device basis. Would this cover your use-case and work in practice?
You can try this out immediately using User-defined Overrides. The below Knowledge Base article goes over the steps for doing this. Keep in mind that for E-series, the CLI commands entered are based on the WLAN index, so it is important that they match the WLAN ordering you set in the AP Group.
Discussion of what WLANs Overrdes to include for both R-series and E-series is being held in the below thread. Please add to the discussion if there are any specific parameters that you feel should be included.