After some investigation I've worked out a way to backup Ubiquiti AirOS configurations via Solarwinds. Putting the details in this post for anyone else who will find this useful.
For anyone else who also needs to utilise this functionality with NCM copy the below text (between the ---------- lines) into notepad and save as file name "Ubiquiti AirOS-1.3.6.1.4.1.9.1069.ConfigMgmt-Commands" in your Solarwinds DeviceTypes folder (c:\Program Files (x86)\Solarwinds\Configuration Management\DeviceTypes).
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<!--SolarWinds Network Management Tools-->
<!--Copyright 2005 SolarWinds.Net All rights reserved-->
<Configuration-Management Device="Cisco WLC" SystemOID=" 1.3.6.1.4.1.9.1.1069">
<Commands>
<Command Name="Reset" Value=""/>
<Command Name="PreCommand" Value="y"/>
<Command Name="MenuBased" Value="false"/>
<Command Name="DownloadConfig" Value="cat /tmp/system.cfg"/>
</Commands>
</Configuration-Management>
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You will also need to create a connection profile under "Admin -> Settings -> NCM Settings -> Manage Connection Profiles" as Ubiquiti AirOS doesn't support RADIUS/TACACS auth at this stage:
When adding the device into NCM select your Global Connection Profile created above and set the Device Template to "Ubiquiti AirOS".
Hope this helps anyone else who has run into this issue also.