I just came across your question. I have been working with nps in windows 2008/2008r23 alot recently. If you haven't found a good solution for this problem I might have one.
Decide on which of your servers is going to be your master. Make your chances on this server. Using powershell you can export the configuration for nps including shared secrets to a config file. It will not include sql database accounting setup. You can save it on a shared location between your servers. Then on the slave server (your servers that you want to apply your changes to) you can run the powershell import. You can script the export and import on your servers.
Export Config from powershell prompt on master server
netsh nps export filename = "C:\config.xml" exportPSK = YES
Import Config from powershell prompt on slave server
netsh nps import filename = "C:\config.xml"
You can script this and also substitute any shared storage you have between the servers. With powershell you can copy the file over to your remote servers with
copy-item C:\Config.xml \server\share\Config.xml
Potentially you could take these pieces and with a little more powershell and have a script that you run whenever you make a change that will push out the change to all of your nps servers.