I've had "ask competition neutral (exclude home)" work. Either it will take a list of stadia from the country, or if the stadium pool has at least 3 stadia (so that there's always one available that's not home or away). I think that's probably how they did the old Spanish Cup finals where it was a neutral ground each time.
If you don't specify a stadium pool, stadium_0 is just the biggest in the country IIRC (that fits whatever requirements that tournament has for capacity, condition,etc)