In KSP 1.2.2 there is an extension for GameEvents that allows mods to register their own GameEvents and for any Mod to find and register a callback for said GameEvents.
How to use (example):
Define your GameEvent as a static in Mod A:
public static EventData<Part, ProtoCrewMember> onKerbalFrozen;
and Initialize it somewhere early before ModB tries to find and register it.
onKerbalFrozen = new EventData<Part, ProtoCrewMember>("onKerbalFrozen");