I found another forum post that had the answer. I had to use Google Translate 
because it wasn’t in English, but it helped me to get it figured out. For anyone else struggling with this here is what worked for me.
In my case I have a MASTER PlaySMS instance running on a virtual machine in my datacenter, and two SLAVE PlaySMS instances running on Raspberry Pi’s, each with two Huawei E353 modems in them. The slaves only need to be able to reach the master via HTTP. The master does not need to connect to the slaves.
On the MASTER: (the master URL in the example is http://playsms.id)
Go to Settings -> Manage gateway and SMSC - Add the Playnet SMSC
Fill in the fields below and save:
SMSC name: e.g. PlaynetMaster
Local playnet username: e.g. playnet-user
Local playnet password: password for the username above, e.g. pw67675H
Go to Settings -> Main configuration: Set the Default SMSC to the name of the Playnet SMSC.
On the SLAVE:
Modify the slave (there can be more than one, the example here uses one slave only):
Go to Settings -> Manage gateway and SMSC - Add the Playnet SMSC
Fill in the fields below and save:
SMSC name: e.g. PlaynetSlave1
Remote is on: 1 (don’t use “true” like I did the first time and have it not work for days…set it to the string value “1”)
Remote playSMS URL: e.g. http://playsms.id
Remote playnet SMSC name: PlaynetMaster
Remote playnet username: Local username in the master playnet (e.g. playnet-user)
Remote playnet password: is the password in the master Local playnet (e.g. pw67675H)
Send SMS from remote using local username Local username on the slave to send with (e.g. admin)
Go to Settings -> Route incoming SMS -> Post rules -> Forward all sandbox SMS to a URL:
http://playsms.id/index.php?app=ws&op=playnet&go=set_incoming&smsc=PlaynetMaster&u=playnet-user&p=pw67675H&payload={SANDBOX_PAYLOAD}
The above is an example when playSMS Master Remote URL is http://playsms.id
SANDBOX_PAYLOAD payload = {} is mandatory
In this instance the MASTER SMSC (smsc) is PlaynetMaster, username (u) is playnet-user, password § is pw67675H
TESTING:
As long as sending is working on the slaves…
You should be able to create a new message on the MASTER and have one of the SLAVES retrieve it and send it automatically.
Incoming SMS messages on the SLAVES should get forwarded to the MASTER.
Again, hope this helps someone out. I’ll try to get all the details written up in a blog post in the next week or two and I’ll put a link here when that’s ready.
Matt