For minecraft PC edition, it's possible to forward by adding a SRV record to defined DNS server, because PC edition fetches from DNS for the real address and port of server. That make it possible to build a server network. But it don't work on Pocket Edition.
Example on SRV forwarding:
s34.example.com:19132 -> DNS (SRV record added) -> h1.example.com:20991
Adding the support in SRV forwarding of MCPE is an enhancement who make opening servers easier, to attract more and more people playing on minecraft PE.
(For the devices, I tested it won't work because I don't know the key word for SRV forward, for example "_udp._minecraftpe")