Hi Markus,
I took a look at the MIB, and while I found the ServiceLicense OIDs, a service license isn't the same as the "base" or "feature" licenses. I'm honestly not sure what the service license is used for. I looks like it was used with older versions of EXOS and should be updated or removed in the MIB.
It might be best if you can open a ticket with GTAC to have engineering take a look at the possibility of adding or repairing this capability.