Unfortunately this is not possible, but there is also a good technical reason why it isn’t 🙂
In runtime, SubGraphs do not initialize until they are first used (unless Pre-Initialize Subgraphs is checked in GraphOwner insepctor). As a result, when entering playmode, if the subgraph in graph editor was to be remain visible, then it would mean that we will no longer be viewing the initialized instance subgraph that is used in runtime, but rather still viewing the original asset subgraph. This can be totally confusing since we will be viewing the wrong graph (not to mentioned that accidentally also modify it).
I hope this clarifies the technical reason behind this 🙂