Unfortunately, this can't be done with structural search and replace. For one thing, there is no construct to match against the absence of something, so there's no way to match against a method invocation that does NOT have an assignment of its return value.
As you note, there are inspections that track pure functions that don't use the return value, and they're not implemented with SSR. You can make them apply to your methods by applying the [Pure]
attribute to them. However, this is implying that the method actually is pure, i.e. has no side effects, so may be the wrong semantic in this instance.