The expression parser isn’t fully compiled C++ code, it’s a unique little custom-made parser that can essentially assemble and run a simple math expression. I think it can only contain UPROPERTYs and basic C++ operators.
Storing the array’s Num() into a dummy UPROPERTY variable and then reading that variable in the EditCondition will work, if you can figure out how to update the dummy variable. On an actor you can use PostEditChangeProperty(…) for this.
You can wrap the dummy variable in #if WITH_EDITORONLY_DATA to keep some of the silliness out of shipping builds.