I have a similar rule, but instead of opening fire the AT guns fight in every round and can be taken as casualties. They still only hit on 1, but each hit can be applied to an enemy tank.
When my firends and I play A&AR we use a house rule for anti-tank guns like yours, except we use AA guns.
At the beginning of a combat in a territory with an AA gun, instead of using it to fire at aircraft, you can instead chose to fire it against tanks
Roll one dice for every tank and on a 1 there hit.
You may want to take your original AT gun rules out to replace with this one, because it is more realistic,
The 88 was not deisigned as an artillery piece, but as a anti-aircraft gun, but later it was discovered that it was a good AT gun too.
so it is only logical that in an A&A Africa game, AA guns can also be used as AT guns.