Thanks for your help Vishal, in the end I have just loaded the rlm status output into an array using powershell and referenced the element for the firmware version. This gets me what I need for the time being.
I would prefer to be accessing this info with a proper API call through a powershell cmdlet. Would you know if I could feed this back somewhere as an addition to a wishlist for pipeline work?
There doesn't appear to be an API for getting the RLM version. As Vishal pointed out, system-cli is an unsupported API. However, sending CLI commands via SSH is fully supported.
Although not as good as an actual API for RLM details, the forthcoming Toolkit 1.3 will do this:
PS C:\> Invoke-NaSsh rlm status
Remote LAN Module Status: Online Part Number: 110-00057 Revision: D0 Serial Number: 391066 Firmware Version: 1.3 Mgmt MAC Address: 00:A0:98:06:E6:CA Ethernet Link: down Using DHCP: no IPv4 configuration: IP Address: 192.168.33.107 Netmask: 255.255.248.0 Gateway: 192.168.32.254
1.3 release date is "soon" but not yet finalized, so no use asking!