Based on what you are describing, that sounds like a valid solution.
Another solution would be to ensure that the targeted banners are in exclusive campaigns, but the default banner is in a remnant campaign.
Also, another other solution would be to put all your geo-targeted banners into one zone, and the default banner into a second zone, and the zone-chain the two zones.
That way, if a user visits from a country that has a banner which can be shown in the first zone (full of banners that are targeted to specific countries), then they will see the banner for their country.
However, as all the banners in the first zone are targeted, if they are not from any of the countries listed, then no banner will be able to be shown - and then it will chain through to the second zone, and they will see the default banner there.
Either way, this would save you from having to target the default banner to only deliver when users are NOT from the target countries.