SSAS Tabular QueryMode Confusion. Why Hybrid mode is useful?
You are correct that currently (up through SSAS 2017 Tabular) is is not possible to have mixed mode partitions. The whole model can operate in DirectQuery or In-Memory. The DirectQueryMode connection strong property can specify whether SSAS should use DirectQuery or In-Memory for answering the current query. But that's not honestly too much better than just deploying the model to two different databases, one DirectQuery and one In-Memory. 

There is a similar connection string DataView=Sample which has SSAS answer the query using a special cached partition marked as the sample partition. 


But this is still not what you want because a particular query won't union results from DirectQuery partitions and sample In-Memory partitions. 

I will try to update this answer if a future release of SSAS adds the feature you are wanting.