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!