The syntax problem with "/" for subnets seems a problem of MyDNSConfig, and NOT of MyDNS. In fact, when modifying directly the table dns_soa in MyDNS Database, inserting a zone "aaa/fffzzz.yyy.xxx.in-addr.arpa." is obviously possible and this works with the IPS adopting that syntax for subnets reverse DNS requests.
Now the problem is that you have to modify this zone from the DB. If MyDNSConfig developers can modify the code, it would be nice